Basic analog telephone service supporting full duplex (FDX) conversational voice communications and operator services. See also basic service and PANS.
(Plain Old Telephone System or Service) See PSTN and AMPS.
A Century Ago
At the turn of the 20th century, the "plain old telephone system" was neither plain nor old. Instruments such as this Blake wall phone were very high-tech contraptions. (Image courtesy of Nortel Networks.)