Plain old telephone service (POTS) is the voice-grade telephone service that remains the basic type of housing as well as undersized company service connection to the telephone network in most parts of the globe. The given name is a retronym, as well as is an indication of the telephone service still accessible after the arrival of more highly developed forms of telephony for example ISDN, mobile phones as well as VoIP. POTS has been accessible almost as the preface of the public telephone system in the late 19th century, in a type mostly unaffected to the normal user in spite of the introduction of Touch-Tone dialing, electronic telephone exchanges as well as fiber-optic communication into the public switched telephone network (PSTN).

The system was at first well-known as the Post Office Telephone Service or else Post Office Telephone System in several countries. The phrase was dropped as telephone services were detached from the control of nationwide post offices.

POTS services include:

 bi-directional or else full duplex, voice path with inadequate frequency varies from 300 to 3400 Hz: in further words, a signal to transmit the sound of the human voice both ways at once;
 call-progress tones, for example dial tone as well as ringing signal;
 Subscriber dialing;
 Operator services, for example directory assistance, long distance, with conference calling assistance;
 A standards compliant analog telephone interface with BORSCHT functions

In U.S, the couple of wires from the mid switch office to a subscriber's home is called a subscriber loop. It is normally power-driven by −48V direct current (DC) as well as backed up by a huge bank of batteries (linked in series) in the central office, resultant in continuance of service throughout most viable power outages. The subscriber loop normally carries a "load" of about 300 Ohms, as well as does not pose a threat of electrocution to human beings (even though shorting the loop can be felt as a disagreeable sense).

Lots of calling features develop into obtainable to POTS subscribers after automation of telephone exchanges all through the 1970s as well as 1980s. The services consist of:

 Voicemail
 Caller ID
 Call waiting
 Speed dialing
 Conference call (three-way calling)
 Enhanced 911
 Centrex
 And additional services.

The communications circuits of the PSTN carry on to be updated by advances in digital communications; but, other than improving noise quality, these changes have been mostly translucent to the POTS consumer. In most cases, the purpose of the POTS local loop accessible to the client for connection to telephone equipment is almost unaffected as well as remains well-suited with old Pulse dialing telephones, even ones dating back to the early 20th century.

Due to the extensive accessibility of POTS, latest forms of communications devices for example modems as well as facsimile machines are intended to make use of POTS to broadcast digital information.