The Supreme Court of Pennsylvania has outlined the prohibitive reach of the wiretap statute as encompassing mechanical invasions over telephone extensions and listening on an extension without the consent of the communicating parties.
Commonwealth v. Murray, 423 Pa. 37, 223 A.2d 102 (1966).
David J. Pleva,
Evidence - Wiretapping - Extension Telephones,
Duq. L. Rev.
Available at: https://dsc.duq.edu/dlr/vol5/iss3/12