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Plain old telephone service, or POTS, is a term which describes the voice-grade telephone service that remains the basic form of residential and small business service connection to the telephone network in most parts of the world. The name is a reflection of the telephone service still available after the advent of more advanced forms of telephony such as ISDN, mobile phones and VoIP. It has been available almost since the introduction of the public telephone system in the late 19th century, in a form mostly unchanged to the normal user despite the introduction of Touch-Tone dialing, electronic telephone exchanges and fiber-optic communication into the public switched telephone network (PSTN).

The system was originally known as the Post Office Telephone Service or Post Office Telephone System in many countries. The term was dropped as telephone services were removed from the control of national post offices.

POTS services include:

• bi-directional, or full duplex, voice path with limited frequency range of 300 to 3400 Hz: in other words, a signal to carry the sound of the human voice both ways at once;
• call-progress tones, such as dial tone and ringing signal;
• subscriber dialing;
• operator services, such as directory assistance, long distance, and conference calling assistance;
• a standards compliant analog telephone interface including BORSCHT functions

In the United States, the pair of wires from the central switch office to a subscriber's home is called a subscriber loop. It is typically powered by -48V direct current (DC) and backed up by a large bank of batteries (connected in series) in the central office, resulting in continuation of service during most commercial power outages. The subscriber loop typically carries a "load" of about 300 Ohms, and does not pose a threat of electrocution to human beings (although shorting the loop can be felt as an unpleasant sensation).

Many calling features became available to POTS subscribers after computerization of telephone exchanges during the 1970s and 1980s. The services include:

• Voicemail
• Caller ID
• Call waiting
• Speed dialing
• Conference call (three-way calling)
• Enhanced 911
• Centrex

among other services.

The communications circuits of the PSTN continue to be modernized by advances in digital communications, however, other than improving sound quality, these changes have been mainly transparent to the POTS customer. The function of the POTS local loop presented to the customer for connection to telephone equipment is practically unchanged and remains compatible even with telephones built in the early 20th century.

Due to the wide availability of POTS, new forms of communications devices such as modems and facsimile machines are designed to use the POTS service to transmit digital information.

While POTS provides limited features, low bandwidth and no mobile capabilities, it does provide greater reliability than other telephony systems (mobile phone, VoIP, etc.). Many telephone service providers attempt to achieve "dial-tone availability" more than 99.999% of the time the telephone is taken off-hook. This is equivalent to having no dial-tone available less than five minutes each year, and was an often-cited benchmark in marketing and systems engineering comparisons, the so-called "five nines" reliability standard.

• Public switched telephone network
• RJ11 - the type of telephone jack common in North America for single line POTS telephones
• DTMF - the tones used for pushbutton dialing
• 25-pair color code
• Voice-grade copper
• Twisted pair
• Foreign exchange office (FXO)
• Foreign exchange station (FXS)
• Telephone network box

• Doing your own telephone wiring