Smith v. Maryland (1979)
a. Facts: McDonough was robbed and began receiving threatening phone calls from a man identifying himself as the robber. The police used a pen register (“trace”) to monitor the calls and record the numbers. The defendant argues that he has a reasonable right to privacy in the numbers he dials and any recording of such numbers is a search in violation of that right.
b. Holding: 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.
c. Reasoning: Although the conversations that you have on the phone are protected, there is no reasonable expectation of privacy in the phone numbers you dial and receive calls from. This is information that he voluntarily turns over to a third party (similar to Greenwood reasoning).
d. Note: A similar reasoning applies to bank records. The court has found no reasonable expectation of privacy because you are willingly transacting with a third party, the bank.
e. Statute: In 1986, Congress enacted a statute that now prohibits the installation or use of pen registers without a court order.