Plain old telephone service
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Plain old telephone service, or rarely publicly offered telephone service, is a retronym for voice-grade telephone service that employs analog signal transmission over copper loops.
In many countries, the post office originally operated telephony systems: hence ubiquitous "PO" markings on historical telephone infrastructure in many countries
DTMF signaling
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Dual-tone multi-frequency signaling is a telecommunication signaling system using the voice-frequency band over telephone lines between telephone equipment and other communications devices and switchi
Plain old telephone service
…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). POTS was the standard service…
The Western Electric 2500, a typical late 20th century American touch-tone telephone
Autovon keypads were a typical application for use of all sixteen DTMF signals. The red keys in the fourth column produce the A, B, C, and D DTMF events.
Two CMD CM8870CSI DTMF receivers