Extension (telephone)

extensionextensionstelephone extensionextension numberextension phoneextension telephonephone extensiontelephone extensions
In residential telephony, an extension telephone is an additional telephone wired to the same telephone line as another.wikipedia
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Business telephone system

PBXprivate branch exchangePABX
Some telephones intended for use as extensions have built in intercom features; a key telephone system for a small business may offer two to five lines, lamps indicating lines already in use, the ability to place calls on 'hold' and an intercom on each of the multiple extensions. In business telephony, a telephone extension may refer to a phone on an internal telephone line attached to a private branch exchange (PBX) or Centrex system. A hotel or motel, 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 which are presented to the guest upon check-out.
Each device connected to the PBX, such as a telephone, a fax machine, or a computer modem, is referred to as an extension and has a designated extension telephone number that may or may not be mapped automatically to the numbering plan of the central office and the telephone number block allocated to the PBX.

Off-premises extension

Off Premise Station
An off-premises extension, 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.
An off-premises extension (OPX), sometimes also known as off-premises station (OPS), is an extension telephone at a location distant from its servicing private branch exchange (PBX).

Direct inward dial

Direct Inward Dialingdirect inbound dialingDDI
For inbound calls, a switchboard operator or automated attendant may request the number of the desired extension or the call may be completed with direct inbound dialing if outside numbers are assigned to individual extensions.
The PBX may use this information to route the call directly to a telephone extension within the organization without the need for an operator or attendant.

Switchboard operator

telephone operatoroperatortelephonist
For inbound calls, a switchboard operator or automated attendant may request the number of the desired extension or the call may be completed with direct inbound dialing if outside numbers are assigned to individual extensions.
As well as the people that were employed by the public networks, operators were required at private branch exchanges (PBX) to answer incoming telephone calls and connect them to the correct extension.

Trunking

trunktrunk linetrunks
Since not all users dial out at the same time, sharing trunk lines is cheaper than giving each phone its own private landline.
When needed they can be used by any telephone connected to the PBX, while the station lines to the extensions serve only one station telephones.

Telephone

phonetelephonesLocal Telephone Service
In residential telephony, an extension telephone is an additional telephone wired to the same telephone line as another.

Telephone line

phone linelinetelephone cable
In residential telephony, an extension telephone is an additional telephone wired to the same telephone line as another. In business telephony, a telephone extension may refer to a phone on an internal telephone line attached to a private branch exchange (PBX) or Centrex system.

Intercom

Intercom Systemintercom systemsCall button
Some telephones intended for use as extensions have built in intercom features; a key telephone system for a small business may offer two to five lines, lamps indicating lines already in use, the ability to place calls on 'hold' and an intercom on each of the multiple extensions.

Hold (telephone)

holdon holdcall hold
Some telephones intended for use as extensions have built in intercom features; a key telephone system for a small business may offer two to five lines, lamps indicating lines already in use, the ability to place calls on 'hold' and an intercom on each of the multiple extensions.

Centrex

Integrated Business Network
In business telephony, a telephone extension may refer to a phone on an internal telephone line attached to a private branch exchange (PBX) or Centrex system.

Telephone numbering plan

Area codeTelephone codearea codes
The PBX operates much as a community switchboard does for a geographic telephone numbering plan and allows multiple lines inside the office to connect without each phone requiring a separate outside line.

Automated attendant

auto attendantauto-attendantphone tree
For inbound calls, a switchboard operator or automated attendant may request the number of the desired extension or the call may be completed with direct inbound dialing if outside numbers are assigned to individual extensions.

Leased line

leased linesInternational Private Leased Circuitleased
An off-premises extension, 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

VoIPVoice over Internet Protocolvoice-over-IP
Voice over IP 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.

Virtual private network

VPNvirtual private networksVPNs
Voice over IP 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.

Telephone number

phone numbertelephone numbersphone numbers
Extensions make it easier for workers to bring their telephone number with them if they change offices in the building.

Automatic message accounting

Centralized Automatic Message Accountingautomatic message accountcall accounting
Extensions allow personalized traffic and call accounting data to be captured by the PBX or a phone recorder for cost control.

Hotel

hotelsluxury hotelhotel chain
A hotel or motel, 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 which are presented to the guest upon check-out.

Motel

motelsTourist Homemotor court
A hotel or motel, 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 which are presented to the guest upon check-out.

SIP URI scheme

SIP addresssips
A SIP address is a URI that addresses a specific telephone extension on a voice over IP system.

IBM 1750, 2750 and 3750 Switching Systems

IBM 1750IBM 1750, 2750 and 3750 Switching SystemIBM 2750 and 3750 Switching Systems
Extension, trunk and tie lines were connected by discrete transistors on plug-in panels.

Dialexia

Dialexia software enables users to connect multiple phones (e.g., extensions, ring groups, etc.), share lines among several phones and implement business PBX telephone phone features such as voicemail, caller ID, call forwarding & call recording into their virtual PBX.

Call parking

Call parkpark
This transfers the current telephone conversation to an unused extension number and immediately puts the conversation on hold.

Ringback number

ringbackplant test numbers
The verification tone is added so that telephone subscribers cannot easily use the ringback system as an intercom, to call other extensions of their own phone line by simply initiating a ringback test and waiting for the phone to stop ringing and converse when someone else on their own line has answered the extension.