In the majority opinion, Justice Harry Blackmun rejected the idea that the installation and use of a pen register constitutes a violation of the suspect's reasonable expectation of privacy since the telephone numbers would be available to and recorded by the phone company anyway.

[3] That ruling was overturned in 1967 in Katz v. United States, in which the Supreme Court held that the Fourth Amendment applied not just to places but to a person's private life, thus making a warrant required for many more types of police searches.

[1] Smith appealed his conviction in Maryland court, arguing that police collection of information about his telephone calls was a search that required a warrant under the Fourth Amendment.

"[1] In another dissenting opinion, Justice Thurgood Marshall expressed his disagreement with the majority's interpretation of the third-party doctrine, stating: "The use of pen registers, I believe, constitutes such an extensive intrusion.

[6] However, the ruling concerned landline telephone technologies that were prevalent in 1979, but the precedent is still being used decades later to justify government surveillance of more advanced Internet communications platforms that collect much more personal information, including the content of messages, and are not necessarily used "voluntarily".