Plain old telephone service

Plain old telephone service (POTS), or rarely publicly offered telephone service, is a retronym for voice-grade telephone service that employs analog signal transmission over copper loops. The term encapsulates a technology that has been available since the introduction of the public telephone system in the late 19th century, remaining largely unchanged despite the introduction of innovations such as Touch-Tone dialing, electronic telephone exchanges and fiber-optic communication into the public switched telephone network (PSTN).

Source: Wikipedia — Plain old telephone service (CC BY-SA 4.0)