Murphy, C. J.,
delivered the opinion of the Court. Digges, Eldridge and Cole, JJ., dissent. Eldridge, J., filed a dissenting opinion in which Digges, J., concurs at page 174 infra. Cole, J., filed a dissenting opinion at page 178 infra.
Whether electronically obtained evidence was improperly admitted at the appellant Smith’s criminal trial in violation of state law and the federal constitution is the central issue in this case.
Smith was charged with having robbed Patricia McDonough on March 5, 1976. Evidence adduced at the trial showed that the victim was returning to her home shortly after midnight on the morning of the crime when she observed a man in her neighborhood changing a tire on a 1975 Monte Carlo automobile which had a dark green bottom and a tan top. As Miss McDonough approached her home, she was suddenly grabbed from behind and her pocketbook forcibly [158] taken from her. In the course of the robbery, the victim had a full-face view of the robber and promptly gave Officer Kenneth Lucas a description of her assailant and of the 1975 Monte Carlo automobile.
Shortly after the crime was committed, Miss McDonough received a threatening and obscene telephone call from an individual who identified himself as the person who had robbed her. She thereafter received a series of such calls from the robber and so advised the police. Unknown to the police, a friend of Miss McDonough, Walt Heline, had attached a recording device to her telephone and instructed her how to tape the robber’s conversation when he called. After Miss McDonough taped three or four calls from her assailant, she informed the police that she had recorded the conversations, and eventually gave the tapes to them.
In the meantime, on March 13, at the request of the police, the telephone company, at its central office, installed terminating accounting equipment on the victim’s telephone line in an effort to determine the origin of the calls she was receiving from the robber. As a result, it was ascertained that some of the calls were being made from pay phones in the immediate vicinity of the victim’s home. Earlier, the victim had advised the police that she thought one of the calls had been made from a telephone at a private residence.
On March 15, Miss McDonough received a call from the robber requesting that she step out on her porch so that he could see her. She did so and observed the 1975 Monte Carlo which she had earlier described to the police, driving slowly by her home.
Officer Lucas, to whom the victim had originally reported the crime, was on'the lookout for a man fitting the description of the robber and of the described vehicle. On March 16, in the general vicinity of the victim’s home, the appellant Smith stopped Lucas and sought his assistance in opening the locked door of his 1975 Monte Carlo. Lucas took the license number of the vehicle, learned that it was registered to the appellant Smith, and so notified other investigating police officers.
On March 17, the telephone company, at the request of the [159] police, installed a pen register1 at its central offices to record the phone numbers of calls made from the telephone at Smith’s residence. On March 17, a call was made from Smith’s residence to the victim’s home. The police thereafter obtained a search warrant to search Smith’s automobile and residence. The search of the residence revealed that a page in Smith’s telephone book was turned down; it contained the name and number of the victim. On March 19, the victim viewed a six-man line-up at police headquarters and identified the appellant Smith as the man who robbed her.
In pretrial motions, Smith had sought to suppress the evidence obtained by the tape recordings and the pen register; he also moved to suppress the line-up identification. He contended that the attachment of the recording device to the victim’s telephone without a court order violated Maryland Code (1957, 1976 Repl. Vol.) Art. 27, § 125A (a); under that section, it is a misdemeanor “for any person in this State to use any electronic device ... to overhear or record any part of the conversation or words spoken to or by any person in private conversation without the knowledge or consent... of that other person.” He also contended that the recording device attached to the victim’s phone violated Code (1974), § 10-402 of the Courts and Judicial Proceedings Article; that section prohibits a person from obtaining “the whole or any part of a telephonic ... communication to which the person is not a participant by means of a device ... unless consent is given by the participants.” Appellant further contended that the evidence resulting from the installation of the pen register should be suppressed because its obtention was based on information gleaned from the unlawful tape recordings of the telephone conversations. He also argued [160] that the pen register constituted an unlawful “interception” of a telephonic communication forbidden by § 10-402 of the Courts Article. He furthermore maintained that, absent á court order or search warrant, the use of the pen register constituted an illegal search and seizure in contravention of the fourth amendment to the federal constitution. Finally, Smith argued that without the illegally obtained electronic evidence he would not have been arrested, required to appear in a line-up and identified by the victim. He therefore claimed that the line-up identification should also be suppressed, but he withdrew this contention before the trial judge acted on his motions.
The trial judge overruled the motions to suppress, and the electronically obtained evidence was admitted. Smith was found guilty of robbery and sentenced to ten years in prison. We granted certiorari prior to decision by the Court of Special Appeals to review the important issues raised in the casé.
(1)
The Tape Recorded Telephone Conversations
At the trial, the State conceded that the recording of the telephone conversations violated § 125A of Art. 27. It maintained that the tape recordings were nevertheless admissible in evidence because the only sanction prescribed by the statute was criminal prosecution of those who violate its provisions. The Court of Special Appeals so held in Reed v. State, 35 Md. App. 472, 372 A. 2d 243 (1977), and Pennington v. State, 19 Md. App. 253, 310 A. 2d 817 (1973), cert. denied, 271 Md. 742, cert. denied, 419 U. S. 1019 (1974). The appellant does not challenge that interpretation of the statute, and we therefore have no occasion to consider the question in this case.
Appellant claims instead that the attachment to the victim’s phone of the recording device without a court order constituted an illegal “interception” of a telephonic communication in contravention of § 10-402 of the . Courts Article.
[161] Until its repeal by ch. 692 of the Acts of 1977,2 § 10-402 was part of the Maryland Wire Tapping Act, §§ 10-401 through 10-408 of the Courts Article, in effect at the time of the appellant’s arrest and prosecution. That Act declared in § 10-401 that the right of the people to be secure against “unreasonable interception of telephonic ... communications may not be violated.” It expressed the legislative mandate that the “interception and divulgence of a private communication by any person not a party thereto is contrary to the public policy of the state, and may not be permitted except by court order in unusual circumstances to protect the people.” Section 10-402 (a) makes it unlawful, absent a court order, for any person to obtain a telephonic communication to which he is not a participant by means of any device unless consent is given by the participants. Section 10-406 provides that evidence obtained in violation of the Maryland Wire Tapping Act is inadmissible in court.
The appellant relies on Robert v. State, 220 Md. 159, 151 A. 2d 737 (1959), as authority for the exclusion of the tape recordings under § 10-402 (a). In Robert, police officers, anticipating that the defendant would make a phone call to certain friends in a motel, positioned themselves at the motel’s telephone switchboard. When the expected call came through the switchboard, the officers monitored it by means of a headset connected through a press key to the switchboard. After observing that the officers could not be classified as participants in the conversation, and that they overheard it without the consent of all of the participants, our predecessors held that the headset was an electrical device by which the officers obtained the telephone conversation in contravention of the Act’s provisions, rendering the evidence thereby obtained inadmissible in court.
[162] Robert is plainly inapposite on its facts. There, the police officers were not participants in the conversation. In the present case, Miss McDonough was a participant in the conversations , which she recorded. There is no requirement in § 10-402 (a) that consent to the recording must be given by all participants in the conversation. Consequently, there was no violation of § 10-402 (a), although plainly the recording of the conversations violated Art. 27, § 125A. Cf. Clark v. State, 2 Md. App. 756, 237 A. 2d 768 (1968), cert. denied, 394 U. S. 1001 (1969).
Appellant’s suggestion that it was Heline and not the victim who recorded the conversations is not supported by the record. Nor is there any evidence to support Smith’s claim that in attaching the recording device to the victim’s phone Heline acted as a police agent. Simply because the police learned, after the fact, that the device had been attached to the victim’s phone, but did not require its removal, does not warrant a finding that § 10-402 (a) was violated. Finally, there is no justification for Smith’s reliance on Commonwealth v. McCoy, 442 Pa. 234, 275 A. 2d 28 (1971), and Cameron v. State, 365 P. 2d 576 (Okla. 1961), to establish that § 10-402 (a) was violated by the recording of the conversations; the statutes involved in those cases were markedly different from § 10-402 (a) and therefore are not applicable in this case.
(2)
The Pen Register
We find no merit in the argument that the installation of the pen register at the central offices of the telephone company to record the phone numbers of outgoing calls made from Smith’s residence telephone constituted the “interception” of a telephonic communication in violation of § 10-402 (a).
The Supreme Court held in United States v. New York Telephone Co., 434 U. S. 159, 98 S. Ct, 364, 54 L.Ed.2d 376 (1977), that a pen register is not encompassed within the provisions of Title III of the Omnibus Crime Control and Safe Streets Act of 1968, 18 U.S.C. §§ 2510-2520. That Act, which [163] comprehensively regulates wiretapping and electronic surveillance, requires a court order authorizing or approving the interception of a wire or oral communication. The term “intercept” is defined in § 2510 (4) of the statute to mean “the aural acquisition of the contents of any wire or oral communication through the use of any electronic, mechanical, or other device.” The Supreme Court said that pen registers are not within the statute because they are not devices used to intercept oral or wire communications, i.e., they do not “intercept” because they do not acquire the “contents” of a communication, as that latter term is defined in § 2510 (8). The Court said:
“Indeed, a law enforcement official could not even determine from the use of a pen register whether a communication existed. These devices do not hear sound. They disclose only the telephone numbers that have been dialed — a means of establishing communication. Neither the purport of any communication between the caller and the recipient of the call, their identities, nor whether the call was even completed are disclosed by pen registers. Furthermore, pen registers do not accomplish the 'aural acquisition’ of anything. They decode outgoing telephone numbers by responding to changes in electrical voltage caused by the turning of the telephone dial (or the pressing of buttons on push button telephones) and present the information in a form to be interpreted by sight rather than by hearing.” 98 S. Ct. at 369-70.
Although the verbiage of § 10-402 (a) differs from the federal statute, the prohibitions underlying each law require the “interception” of a communication. We conclude, as did the Supreme Court in New York Telephone, and most federal courts which have considered the question,3 that a pen [164] register is not a device which “intercepts” a telephonic communication. Accordingly, the use of the pen register did not violate § 10-402 (a).
Since the evidence procured by recording the telephone conversations which the victim had with her assailant was properly admitted at the trial, Smith’s alternative argument that the pen register evidence must be suppressed as an illegal derivative use of the recorded telephone conversations is also lacking in merit. Cf. Everhart v. State, 274 Md. 459, 337 A. 2d 100 (1975); Carter v. State, 274 Md. 411, 337 A. 2d 415 (1975).
Appellant next contends that pen register surveillance constitutes a search subject to the warrant requirements of the fourth amendment. Since no warrant or court order was obtained authorizing the installation of the pen register, Smith claims that the evidence which the pen register produced, and all evidence derived from its use, must be suppressed. The State, on the other hand, contends that the better-reasoned cases support the view that pen register surveillance is not a search within the fourth amendment and that a warrant is not required to install such a device. Substantial authority exists for each position.
In Katz v. United States, 389 U. S. 347, 88 S. Ct. 507, 19 L.Ed.2d 576 (1967), the Supreme Court held that evidence obtained without a warrant by government agents of words spoken by the defendant in a telephone conversation, which the agents overheard by attaching an electronic listening device to the outside of a public telephone booth from which the defendant had placed a telephone call, violated the fourth amendment. It held, overruling Olmstead v. United States, 277 U. S. 438, 48 S. Ct. 564, 72 L. Ed. 944 (1928), that the fourth amendment governs not only the seizure of tangible items, but also the recording of oral statements overheard, even in the absence of a technical trespass against local property law. It said that the fourth amendment protects people, and not merely places, against unreasonable searches and seizures; that it protects “individual privacy against certain kinds of governmental intrusion”; that what a person knowingly exposes to the public, even in his own home or [165] office, is not a subject of fourth amendment protection; but that what he seeks to preserve as private may be constitutionally protected. The Court said that while the fourth amendment cannot be translated into a general constitutional right to privacy, the activities of the government agents violated “the privacy upon which [the defendant] justifiably relied whilé using the telephone booth and thus constituted a ‘search and seizure’ within the meaning of the Fourth Amendment.” 389 U. S. at 353.
The admissibility of evidence obtained by use of a pen register was considered in United States v. Giordano, 416 U. S. 505, 94 S. Ct. 1820, 40 L.Ed.2d 341 (1974). There, court orders had been issued authorizing interception of wire communications and the installation of a pen register, and these orders were subsequently extended. The application to extend the pen register detailed the contents of conversations intercepted pursuant to the wire interception orders. The Court held that the wire interception orders were invalid since they had not been authorized in conformity with the controlling federal statute. It further held that evidence gathered under the pen register extension order was inadmissible because tainted by the use of the unlawfully intercepted wire communications to secure judicial approval to extend the pen register surveillance order. The Court did not hold that use of a pen register requires compliance with the warrant requirements of the fourth amendment, although that result might be implied in view of the decision to exclude the evidence which the pen register produced.
In a concurring and dissenting opinion by Justice Powell, in which the Chief Justice and Justices Blackmun and Rehnquist joined, it was pointed out that there was no dispute that the pen register order was based on probable cause and lawful under the fourth amendment. In this context, Justice Powell stated: “Because a pen register device is not subject to the provisions of Title III, the permissibility of its use by law enforcement authorities depends entirely on compliance with the constitutional requirements of the Fourth Amendment.” 416 U. S. at 553-554. That Justice Powell would not have decided the constitutional issue is, however, clear [166] from his further statement, appearing at footnote 4, 416 U. S. at 554: “The Government suggests that the use of a pen register may not constitute a search within the meaning of the Fourth Amendment. I need not address this question, for in my view the constitutional guarantee, assuming its applicability, was satisfied in this case.” Justice Powell concluded that the pen register extension order was valid, because based only in part on the unlawfully intercepted wire communications, and that the evidence gathered by the device was admissible.
Relying on Justice Powell’s .statement to support the proposition that the use of a pen register depends on compliance with the requirements of the fourth amendment, the court in Application of U.S. in Matter of Order, Etc., 538 F. 2d 956 (2nd Cir. 1976), stated at 959: “We take this statement to mean that a pen register order involves a search and seizure under the Fourth Amendment, and that a court may issue such an order only upon a showing of probable cause.” A number of other courts have reiterated Justice Powell’s statement concerning pen registers and compliance with the fourth amendment. See United States v. Illinois Bell Tel. Co., 531 F. 2d 809 (7th Cir. 1976); United States v. Doolittle, 507 F. 2d 1368 (5th Cir.), cert. dismissed, 423 U. S. 1008 (1975); United States v. John, 508 F. 2d 1134 (8th Cir.), cert. denied, 421 U. S. 962 (1975); United States v. Brick, 502 F. 2d 219 (8th Cir. 1974). Although each of these cases states that the propriety of the use of a pen register depends upon compliance with the fourth amendment, only Application of U.S. in Matter of Order, Etc. holds that the use of a pen register constitutes a search; indeed, in United States v. John, supra, the court held that it was not necessary to decide that question. In all four of these cases a warrant had in fact been issued, and none of them address the question whether use of the device constitutes a search.
Nor did the Supreme Court decide the question in United States v. New York Telephone Co., supra. It said, 98 S. Ct. at 369: “The Court of Appeals [4] held that pen register [167] surveillance was subject to the requirements of the Fourth Amendment. This conclusion is not challenged by either party, and we find it unnecessary to consider the matter.”
In New York Telephone, the government had obtained a court order, which the telephone company resisted, authorizing the installation of a pen register and directing the telephone company to provide facilities and assistance necessary to install it. At issue in the case was not whether a warrant was necessary; one had been obtained. Rather, the primary issue was whether the District Court had the power to issue an order authorizing pen register surveillance. The Court held that it had such authority under Fed. R. Crim. P. 41 (b) which was sufficiently broad to encompass a search, not limited to tangible items but including electronic intrusions, designed to ascertain the use being made of a telephone. The Court did not hold that the fourth amendment required such an order; it merely said that the District Court had the authority to issue the order.
Under Katz, whether pen register surveillance requires compliance with the fourth amendment depends on whether a telephone subscriber has a constitutionally protected expectation that the numbers which he dials will remain private. In determining whether an expectation of privacy is constitutionally justified, we adopted in Venner v. State, 279 Md. 47, 367 A. 2d 949 (1977), cert. denied, 431 U. S. 932 (1977), the twofold test articulated by Justice Harlan in his concurring opinion in Katz, i.e., “first that a person have exhibited an actual (subjective) expectation of privacy and, second, that the expectation be one that society is prepared to recognize as ‘reasonable.’ ” 389 U. S. at 361. Other courts have followed this test. See, e.g., United States v. Peterson, 524 F. 2d 167 (4th Cir. 1975), cert. denied, 423 U. S. 1088 (1976); United States v. Hitchcock, 467 F. 2d 1107 (9th Cir. 1972), cert. denied, 410 U. S. 916 (1973); Smith v. State, 510 P. 2d 793 (Alas.), cert. denied, 414 U. S. 1086 (1973); People v. Huddleston, 38 Ill. App. 3d 277, 347 N.E.2d 76 (1976).
It is generally held that the expectation of privacy protected by the fourth amendment attaches to the content of a telephone conversation and not to the fact that a [168] conversation took place. Hodge v. Mountain States Tel. & Tel. Co., 555 F. 2d 254 (9th Cir. 1977); United States v. Clegg, 509 F. 2d 605 (5th Cir. 1975); United States v. Baxter, 492 F. 2d 150 (9th Cir.), cert. dismissed, 414 U. S. 801 (1973); United States v. Fithian, 452 F. 2d 505 (9th Cir. 1971); United States v. Harvey, 394 F. Supp. 228 (E.D. Ark. 1975) aff’d 540 F. 2d 1345 (8th Cir. 1976). Clegg dealt with a device in all respects similar to a pen register; it was attached by the telephone company to the defendant’s telephone line to determine whether he was illegally circumventing the telephone company’s billing system by using a so-called “blue box” device to make long-distance calls. The court there said that the fourth amendment “protects only the content of a telephone conversation and not the fact that a call was placed or that a particular number was dialed.” 509 F. 2d at 610. This was so, the court said, “because telephone subscribers have no reasonable expectation that records of their calls will not be made ... [since it is] well known that such records are kept.” Id. at 610. Consistent with Clegg, other courts have held that telephone subscribers have no reasonable expectation that records of their calls will not be made. United States v. Harvey, 540 F. 2d 1345 (8th Cir. 1976); DiPiazza v. United States, 415 F. 2d 99 (6th Cir. 1969), cert. denied, 402 U. S. 949 (1971); United States v. Covello, 410 F. 2d 536 (2nd Cir.), cert. denied, 396 U. S. 879 (1969); Nolan v. United States, 423 F. 2d 1031 (10th Cir. 1969), cert. denied, 400 U. S. 848 (1970); Brandon v. United States, 382 F. 2d 607 (10th Cir. 1967); Baxter, supra; Fithian, supra. These cases in the main involve billing records for toll or long-distance phone calls. They seemingly stand for the proposition that, as against the subscriber’s claim to privacy, the fourth amendment is not applicable to the seizure of such records in the possession of the telephone company because public awareness that the records are routinely maintained negates any constitutionally protected expectation of privacy regarding them.
In Hodge v. Mountain States Tel. & Tel. Co., 555 F. 2d 254 (9th Cir. 1977), the court held that no substantive fourth amendment right was implicated by the warrantless attachment of a pen register to the telephone line of a [169] subscriber suspected of making local obscene calls. In concluding that no constitutionally protected right of privacy was involved, the court saw little practical difference, insofar as public awareness was concerned, between the maintenance of routine telephone billing records and a pen register. It said:
“Although a pen register record differs from telephone company billing records, we have no difficulty in now holding that the information recorded is not protected by the Fourth Amendment.
“A pen register record for a particular telephone contains information different from the telephone company billing records for that telephone. Telephone company billing records show only completed calls, not, as with a pen register, the numbers dialed. Furthermore, a pen register record shows the dialing of telephone numbers which, even if completed, would not be shown by billing records, because the numbers are within a local dialing area. It could be argued that since no records of such calls are normally maintained, an expectation of privacy exists. This admitted difference is not, in our view, of constitutional dimension6 and is more than offset by the fact that pen register records are even farther removed than billing records from the content of the communications. Viewed in the round, the information recorded by pen registers is not entitled to Fourth Amendment protection.”
Judge Hufstedler, in an opinion specially concurring in Hodge, said that the use of the pen register did not constitute a “search” within the meaning of the fourth amendment “because the ‘electronic listening’ does not encroach upon ‘the privacy upon which... [one] justifiably reliefs] ’ ” citing Katz. 555 F. 2d at 266. After noting that there was no justifiable [170] expectation of privacy in the contents of telephone company billing records, she said:
“Similarly, there is no expectation of privacy in the contents of a pén register tape. Like billing records, a pen register tape discloses the numbers dialed from a particular telephone and not the contents of any conversation. In fact, a pen register creates a lesser intrusion into a subscriber’s privacy because, unlike billing records, a pen register tape does not indicate whether any calls were answered.
“True, the telephone company usually does not keep a record of local telephone calls. But most subscribers are unaware of the boundaries of their local dialing zones, especially in cities where these zones do not coincide with traditional geographic boundaries. Furthermore, it is common practice for the telephone company to keep a record of all calls dialed from a telephone which is subject to a special rate structure____ Under these circumstances, subscribers do not harbor any justifiable expectation of privacy that a record will not be kept of their outgoing calls____ (‘... For this reason, the acquisition ... by means of a pen register ... of nothing more than information concerning ... the numbers dialed does not offend the Fourth Amendment.’).” Id. at 266.
The same conclusion was reached in Note, The Legal Constraints Upon the Use of the Pen Register as a Law Enforcement Tool, 60 Cornell L. Rev. 1028, 1044-45 (1975). It was there said:
“[T]he fourth amendment does not bar the use of the pen register. First, even assuming that a privacy expectation is in fact present, it is well settled that toll calls (and their records) are not entitled to a reasonable expectation of privacy. And, with respect to most areas of the country, there seems to be no valid distinction between the expectations associated with local calls on the one hand and those calls that [171] cross the local billing zone on the other hand. The majority of subscribers probably have no real knowledge as to the geographic boundaries of their ‘local call’ zone.”
A second reason for the commentator’s conclusion that warrantless pen register surveillance does not violate the fourth amendment was stated as follows:
“[A]ll telephone subscribers must utilize equipment owned by a third party, the telephone company, in order to place a call. It is therefore unreasonable for a subscriber to assume that the fact of his call passing through the telephone system will remain a total secret from the telephone company. Once this assertion is accepted, it is clear that there can be no reasonable expectation of privacy from law enforcement authorities with respect to the dial pulses detected and recorded by the telephone company. In a variety of analogous contexts, the Supreme Court has determined that a person entitled to receive a communication is similarly entitled to reveal it to government officials without further legal process.” Id. at 1045.
Supportive of the conclusion that pen register surveillance does not violate the fourth amendment is United States v. White, 401 U. S. 745, 91 S. Ct. 1122, 28 L.Ed.2d 453 (1971), and United States v. Miller, 425 U. S. 435, 96 S. Ct. 1619, 48 L.Ed.2d 71 (1976). In White, statements made by the defendant were overheard by government agents by means of a hidden transmitter which an informer agreed to wear during his meetings with the defendant. The Court found no constitutionally protected expectation of privacy that the informant would not simultaneously transmit the conversation to the police. In Miller, the Court held that a bank depositor had no legitimate expectation of privacy in the contents of checks and deposit slips turned over to the bank, stating:
“The depositor takes the risk, in revealing his affairs to another, that the information will be [172] conveyed by that person to the government.... This Court has held repeatedly that the Fourth Amendment does not prohibit the obtaining of information revealed to a third party and conveyed by him to government authorities, even if the information is revealed on the assumption that it will be used only for a limited purpose and the confidence placed in the third party will not be betrayed.” 425 U. S. at 443.
A similar situation exists in the case of telephone calls. While the content of a call is not revealed to the telephone company, the information as to the number dialed must necessarily be revealed, since it is through telephone company switching equipment that calls are completed. As a recipient of such information, the company may reveal it since the caller can have no reasonable expectation that it will remain private. In fact, the caller should have even less of a justified expectation of privacy, since unlike the disclosures in White and Miller the use of a pen register does not reveal the contents of a communication.
Cases involving other types of surveillance are also relevant. In United States v. Hufford, 539 F. 2d 32 (9th Cir.), cert. denied 429 U. S. 1002 (1976), the court held that the installation of an electronic tracking device on a drum of caffeine to aid government agents in tracking the defendant’s truck did not violate the fourth amendment. Citing Katz, the court stated: “[The defendant] did not have a reasonable expectation of privacy as he drove along the public , road. While he hoped that his travel would go unmonitored, his movements were knowingly exposed to the public, and therefore are not a subject of fourth amendment protection.” 539 F. 2d at 33-34. Accord: United States v. Pretzinger, 542 F. 2d 517 (9th Cir. 1976). Contra: United States v. Holmes, 521 F. 2d 859 (5th Cir. 1975), aff’d by an evenly divided court, en banc 537 F. 2d 227 (1976).
The use of mail covers, where postal inspectors copy information contained on the outside of sealed envelopes traveling through the mail, may also be likened to the use of [173] a pen register. In each situation, communications travel through public conveyances; in each the surveillance reveals the destination or point of origin of the communications, but not the content of the message itself. If anything, the use of a mail cover is more of an invasion of privacy than a pen register since the mail cover reveals the identities of the parties. Nonetheless, courts have generally held that the use of mail covers does not violate the fourth amendment. See Lustiger v. United States, 386 F. 2d 132 (9th Cir. 1967), cert. denied, 390 U. S. 951 (1968); Canaday v. United States, 354 F. 2d 849 (8th Cir. 1966). Post-Katz authority upholding the use of mail covers is limited. See United States v. Leonard, 524 F. 2d 1076 (2d Cir. 1975), cert. denied, 425 U. S. 958 (1976) (mail cover on international mail); United States v. Balistrieri, 403 F. 2d 472 (7th Cir. 1968); United States v. Isaacs, 347 F. Supp. 743 (N.D. Ill. 1972). In United States v. Choate, 422 F. Supp. 261 (C.D. Cal. 1976), the court took a contrary view. While it recognized that a person’s expectation of privacy with respect to return addresses on mail is a limited one, the court concluded that a person justifiably expects that the information will be used only for postal purposes and that records of it will not be kept. The court held a person did have a reasonable expectation that his mail would not be used for surveillance purposes. The holding in Choate may be questioned, however, in light of United States v. Miller, supra, where the Supreme Court held that disclosure of information, even on the assumption that it would be used for a limited purpose, negated any expectation of privacy with respect to that information.
We hold that there is no constitutionally protected reasonable expectation of privacy in the numbers dialed into a telephone system and hence no search within the fourth amendment is implicated by the use of a pen register installed at the central offices of the telephone company. While the guarantees of the fourth amendment are broad, they are not boundless, State v. Siegel, 266 Md. 256, 292 A. 2d 86 (1972); not everything a person may want to be private is protected by the fourth amendment. As Katz teaches, the fourth amendment does not afford our citizens “a general [174] constitutional right to privacy.” In not imposing Title III restrictions on the use of pen registers, it is evident, as New York Telephone explicitly points out, that the Congress did not consider that such devices pose a threat to privacy of the same dimension as the interception of an oral communication. As the Supreme Court noted in that case, pen registers do not reveal whether a communication existed and it recognized that such devices are regularly used by the telephone company without a court order “for the purposes of checking billing operations, detecting fraud, and preventing violations of law.” 98 S. Ct. at 373. The intrusion involved in pen register surveillance is minimal; no violation of the integrity of the communication system itself is entailed; and no conversation .is overheard.
Whether a telephone subscriber harbors an actual subjective expectation of privacy in the numbers which he dials is, of course, difficult to know. In all probability, he understands that his calls are placed through mechanical equipment and that some record is made. We think it unlikely that the telephone subscriber distinguishes between local or toll calls with respect to an expectation of privacy in the numbers he dials. Even if he did harbor such an expectation, we are not prepared to say on the record before us that it is one that society would recognize as reasonable and constitutionally protected.
Judgment affirmed; costs to be paid by appellant.