What You Need to Know about the Third-Party Doctrine

In March 1976, a Baltimore woman reported to police that she had been robbed. She provided the police with a description of the robber as well as of a vehicle she believed to be his—a 1975 Monte Carlo. Soon afterwards, she began to receive threatening phone calls from a man identifying himself as the robber. A week and a half after the robbery, police saw a man matching the description provided by the victim driving a 1975 Monte Carlo near the scene of the crime. They noted the license plate number, and found that the car was registered to Michael Lee Smith.

Without seeking a warrant, the police then asked the phone company to install a “pen register” at its offices to create a record of all numbers dialed by Smith. After finding that Smith was indeed calling the victim, police obtained a warrant to search his home, found other evidence of phone calls to the victim, and arrested Smith.

Smith sought to exclude the evidence from the pen register, arguing to the Criminal Court of Baltimore that its use without a warrant violated his Fourth Amendment right against unreasonable searches and seizures. The court, however, found no Fourth Amendment violation. After an appeals court reached the same conclusion, the Supreme Court agreed to hear the case...

http://www.theatlantic.com/technology/archive/2013/12/what-you-need-to-know-about-the-third-party-doctrine/282721/