http://en.wikipedia.org/wiki/Smith_v._Maryland
Smith v. Maryland, 442 U.S. 735 (1979),[1] was a case in which the Supreme Court of the United States held that the installation and use of the pen register was not a "search" within the meaning of the Fourth Amendment, and hence no warrant was required. The pen register was installed on telephone company property at the telephone company's central offices. In the Majority opinion, Justice Blackmun rejected the idea that the installation and use of a pen registry constitutes a violation of the "legitimate expectation of privacy" since the numbers would be available to and recorded by the phone company anyway.
Background
In Katz v. United States (1967), the United States Supreme Court established its "reasonable expectation of privacy" test. It overturned Olmstead v. United States and held that wiretaps were unconstitutional searches, because there was a reasonable expectation that the communication would be private. The government was then required to get a warrant to execute a wiretap.
In Smith v. Maryland, the Supreme Court held that a pen register is not a search because the "petitioner voluntarily conveyed numerical information to the telephone company."
Since the defendant had disclosed the dialed numbers to the telephone company so they could connect his call, he did not have a reasonable expectation of privacy in the numbers he dialed. The court did not distinguish between disclosing the numbers to a human operator or just the automatic equipment used by the telephone company.