In the case of Smith v. Maryland, the U.S. Supreme Court ruled that individuals do not have a reasonable expectation of privacy for information voluntarily given to third parties, thus establishing what is known as the "third-party doctrine." The case involved Michael Lee Smith who was convicted for robbery based on evidence obtained from a pen register installed by police at his phone company without a warrant. A pen register records all numbers called from a particular telephone line. Smith argued this violated his Fourth Amendment rights against unreasonable searches and seizures. However, in its decision, the court held that since people know that they must convey phone numbers to their service provider when making calls and cannot reasonably expect these numbers to remain private, installing such device does not constitute search under Fourth Amendment.

In the dissenting opinion for Smith v. Maryland, Justice Thurgood Marshall argued that individuals have a reasonable expectation of privacy in their telephone numbers and therefore, obtaining these records constitutes a search under the Fourth Amendment. He disagreed with the majority's reliance on Katz v. United States to determine whether an individual has "exhibited an actual (subjective) expectation of privacy," arguing instead that this approach is too narrow and fails to consider broader societal values about what should be private. Furthermore, he contended that people do not voluntarily convey phone numbers to the phone company in any meaningful sense because they must dial those numbers to use their phones - it's not really a choice or voluntary action but rather a necessity of modern life. Therefore, he concluded that police need probable cause and warrant before accessing such information.