Plain Old Telephone Service
Plain Old Telephone Service (POTS) is an industry term that generally relates to telephone service using a minimum (or no) electronics, and having a minimum of features besides voice and perhaps facsimile. It is still compatible with the Public Switched Telephone Network, although the older telephones (e.g., with mechanical dials) may need converters to convert their dialing signals (i.e., Dual Tone Multiple Frequency (DTMF)) to the electronic tones used by the first telephone switch to which they connect.
The term is also used for emergency backup telephones that bypass a complex telephone system that might fail in a disaster. Such a case might have somewhat more electronics, such as an analog telephone using DTMF signaling, which is kept alongside a Voice over Internet Protocol (VoIP) telephone, the latter part of a building system], in a hospital. The basic analog telephone has a separate number and is the last-resort means for the emergency room, surgical suite, command center, etc., to communicate.