United States v. New York Telephone Co.
United States v. New York Telephone Co., 434 U.S. 159 (1977), was a United States Supreme Court case in which the Court held that law enforcement officials may obtain a court order forcing telephone companies to install pen registers in order to record the numbers called from a particular telephone.
Source: Wikipedia — United States v. New York Telephone Co. (CC BY-SA 4.0)