In residential telephony, an extension telephone is an additional

telephone A telephone is a telecommunications device that permits two or more users to conduct a conversation when they are too far apart to be easily heard directly. A telephone converts sound, typically and most efficiently the human voice, into e ...

wired to the same

telephone line A telephone line or telephone circuit (or just line or circuit industrywide) is a single-user circuit on a telephone communication system. It is designed to reproduce speech of a quality that is understandable. It is the physical wire or ot ...

as another. In middle 20th century telephone jargon, the first telephone on a line was a "Main Station" and subsequent ones "Extensions" or even called as intercom. Such extension phones allow making or receiving calls in different rooms, for example in a home, but any incoming call would ring all extensions and any one extension being in use would cause the line to be busy for all users. Some telephones intended for use as extensions have built in

intercom An intercom, also called an intercommunication device, intercommunicator, or interphone, is a stand-alone voice communications system for use within a building or small collection of buildings which functions independently of the public telephon ...

features; a

key telephone system A business telephone system is a multiline telephone system typically used in business environments, encompassing systems ranging in technology from the key telephone system (KTS) to the private branch exchange (PBX). A business telephone syst ...

for a small business may offer two to five lines, lamps indicating lines already in use, the ability to place calls on '

hold Hold may refer to: Physical spaces * Hold (ship), interior cargo space * Baggage hold, cargo space on an airplane * Stronghold, a castle or other fortified place Arts, entertainment, and media * Hold (musical term), a pause, also called a Ferm ...

' and an intercom on each of the multiple extensions. In business telephony, a telephone extension may refer to a phone on an internal

attached to a private branch exchange (PBX) or

Centrex Centrex is a portmanteau of central exchange, a kind of telephone exchange. It provides functions similar to a PBX, but is provisioned with equipment owned by, and located at, the telephone company premises. Centrex service was first installed ...

system. The PBX operates much as a community switchboard does for a geographic

telephone numbering plan A telephone numbering plan is a type of numbering scheme used in telecommunication to assign telephone numbers to subscriber telephones or other telephony endpoints. Telephone numbers are the addresses of participants in a telephone network, re ...

and allows multiple lines inside the office to connect without each phone requiring a separate outside line. In these systems, one usually has to dial a number (typically 9 in North America, 0 in Europe) to tell the PBX to connect with an outside landline (also called DDCO, or ''Direct Dial Central Office'') to dial an external number. Within the PBX, the user merely dials the extension number to reach any other user directly. For inbound calls, a switchboard operator or

automated attendant In telephony, an automated attendant (also auto attendant, auto-attendant, autoattendant, automatic phone menus, AA, or virtual receptionist) allows callers to be automatically transferred to an extension without the intervention of an operator/ ...

may request the number of the desired extension or the call may be completed with

direct inbound dialing Direct inward dialing (DID), also called direct dial-in (DDI) in Europe and Oceania, is a telecommunication service offered by telephone companies to subscribers who operate a private branch exchange (PBX) system. The feature provides service for ...

, if outside numbers are assigned to individual extensions. An

off-premises extension {{Unreferenced, date=December 2009 An off-premises extension (OPX), sometimes also known as off-premises station (OPS), is an extension telephone at a location distant from its servicing exchange. One type of off-premises extension, connected to ...

, where a worker at a remote location employs a telephone configured to appear as if it were an extension located at the main business site, may be created in analog telephony by using a leased line to connect the extension to the main enterprise system.

Voice over IP Voice over Internet Protocol (VoIP), also called IP telephony, is a method and group of technologies for the delivery of voice communications and multimedia sessions over Internet Protocol (IP) networks, such as the Internet. The terms Internet t ...

makes the creation of off-premises extensions inexpensive and trivial as broadband Internet and virtual private networking can extend local network access anywhere in the world. In either system, an off-premises extension is reachable from within the same enterprise simply by calling its extension number directly; for inbound and outgoing calls, it functions as if it were located at the main place of business.

Reasons to use extensions

{{Tone, section, date=December 2020 Since not all users dial out at the same time, sharing trunk lines is cheaper than giving each phone its own private landline.Newton, H: ''Newton's telecom dictionary'', CMP books, 2004, p. 901. Extension to extension calls can be made and kept within the company for cost or security reasons. Extensions make it easier for workers to bring their

telephone number A telephone number is a sequence of digits assigned to a landline telephone subscriber station connected to a telephone line or to a wireless electronic telephony device, such as a radio telephone or a mobile telephone, or to other devices f ...

with them if they change offices in the building. Extensions allow personalized traffic and call accounting data to be captured by the PBX or a phone recorder for cost control. A

hotel A hotel is an establishment that provides paid lodging on a short-term basis. Facilities provided inside a hotel room may range from a modest-quality mattress in a small room to large suites with bigger, higher-quality beds, a dresser, a re ...

motel A motel, also known as a motor hotel, motor inn or motor lodge, is a hotel designed for motorists, usually having each room entered directly from the parking area for motor vehicles rather than through a central lobby. Entering dictionarie ...

, for example, could install PBX extensions in each individual room and use the private branch exchange's call accounting to generate individualised long-distance bills for each room that are presented to the guest upon check-out.

References

Telephony equipment Local loop