The United States Supreme Court has held that the installation and use of a pen register device is not a fourth amendment search requiring judicial authorization.
Smith v. Maryland, 442 U.S. 735 (1979).
Michael P. Weinstein,
Constitutional Criminal Procedure - Fourth Amendment - Search and Seizure - Electronic Surveillance - Pen Register,
Duq. L. Rev.
Available at: https://dsc.duq.edu/dlr/vol18/iss3/14