Business telephone systemwikipedia
A business telephone system is a multiline telephone system typically used in business environments, encompassing systems ranging from small key telephone systems (KTS) to large private branch exchanges (PBX).
PBXprivate branch exchangekey telephone systemPBXsbusiness telephone systemHosted PBXPBXesfollow-mekey phone systemtelephone systems

1A2 Key Telephone System

1A2 Key System1A21A2 key systems
The systems marketed in North America as the 1A, 6A, 1A1 and the 1A2 Key System are typical examples and sold for many decades.
The 1A2 Key Telephone System is a business telephone system developed and distributed by the Western Electric Company for the Bell System.

Telephone exchange

telephone exchangeexchangescentral office
Before the advent of large-scale integrated circuits, key systems were typically composed of electromechanical components (relays) as were larger telephone switching systems. From the 1960s a simulated PBX known as Centrex provided similar features from the central telephone exchange.
For corporate or enterprise use, a private telephone exchange is often referred to as a private branch exchange (PBX), when it has connections to the public switched telephone network.

Trading turret

trading turretTurrets
These systems commonly have their front end units referred to as Turrets and are notable for their presentation of hoot-n-holler circuits.
A trading turret or dealer board is a specialized telephony key system that is generally used by financial traders on their trading desks.

Centrex

centrex
From the 1960s a simulated PBX known as Centrex provided similar features from the central telephone exchange. In wireline telephony, the original hosted PBX was the Centrex service provided by telcos since the 1960s; later competitive offerings evolved into the modern competitive local exchange carrier.
Centrex is a PBX-like service providing switching at the central office instead of at the customer's premises.

Caller ID

caller IDcaller line identificationCLI
A call placed behind a private branch exchange (PBX) has more options.

IP PBX

On-siteprivate branch exchangeIP-PBX
Since the advent of Internet telephony (Voice over IP) technologies, PBX development has tended toward the IP PBX, which uses the Internet Protocol to carry calls.
A IP PBX (IP - Private Branch Exchange) is a system that connects telephone extensions to the Public Switched Telephone Network and provides internal communication for a business.

Extension (telephone)

extensionextensionstelephone extension
Each PBX-connected device, such as a telephone, a fax machine, or a computer modem, is often 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.
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.

Voicemail

voicemailvoice mailvoice-mail
Apart from offering advanced features (like voicemail), the system could also use a VoIP gateway to connect to traditional PSTN lines.
Most cell phone services offer voice-mail as a basic feature; many corporate PBXs include versatile internal voice-messaging services, and *98 Vertical service code subscription is available to most individual and small business land line subscribers.

Voice over IP

VoIPvoice over IPvoice over Internet Protocol
Since the advent of Internet telephony (Voice over IP) technologies, PBX development has tended toward the IP PBX, which uses the Internet Protocol to carry calls. The modern key system is usually fully digital, although analog variants persist and some systems implement VOIP services. Originally having started as an organization's manual switchboard or attendant console operated by a telephone operator or just simply the operator, PBXs have evolved into VoIP centers that are hosted by the operators or even manufacturers.
In 2008, 80% of all new Private branch exchange (PBX) lines installed internationally were VoIP.

Integrated Services Digital Network

ISDNintegrated services digital network ISDN
The modern key system now supports SIP, ISDN, analog handsets (in addition to its own proprietary handsets - usually digital) as well as a raft of features more traditionally found on larger PBX systems.
PRI-ISDN is popular throughout the world, especially for connecting private branch exchanges to the public network.

Attendant console

attendant console
Originally having started as an organization's manual switchboard or attendant console operated by a telephone operator or just simply the operator, PBXs have evolved into VoIP centers that are hosted by the operators or even manufacturers.
An attendant console is a telephone station that is generally part of a private branch exchange (PBX) or Centrex or other private telephone system.

Dial plan

dial plandialing plandialling plan
In addition, a dial plan determines whether additional digit sequences must be prefixed when dialing to obtain access to a central-office trunk.
A dialing plan establishes the permitted sequences of digits dialed on subscriber or station lines with subscriber premises equipment, such as telephones and private branch exchange (PBX) systems.

Competitive local exchange carrier

competitive local exchange carrierCLECCompetitive Local Exchange Carriers
In wireline telephony, the original hosted PBX was the Centrex service provided by telcos since the 1960s; later competitive offerings evolved into the modern competitive local exchange carrier.
Initially, they offered a "shared PBX" service with these switches and interconnected with the ILECs as end users rather than as co-carriers.

Line hunting

hunt grouphunt groupsline hunting
As PBX systems gained popularity, they began to feature services not available in the public network, such as hunt groups, call forwarding, and extension dialing.
Hunt groups are supported by some PBX phone systems.

Direct inward dial

direct inward dialingdirect inward dialdirect 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.

Switchboard operator

telephone operatorswitchboard operatoroperator
Originally having started as an organization's manual switchboard or attendant console operated by a telephone operator or just simply the operator, PBXs have evolved into VoIP centers that are hosted by the operators or even manufacturers. The term PBX originated when switchboard operators managed company switchboards manually using cord circuits.
As well as the people that were employed by the public networks, operators were required at private branch exchanges to answer incoming telephone calls and connect them to the correct extension.

Call parking

parkcall parking
Call park is a feature of some telephone systems that allows a person to put a call on hold at one telephone set and continue the conversation from any other telephone set.

Telephone switchboard

switchboardtelephone switchboardswitchboards
Originally having started as an organization's manual switchboard or attendant console operated by a telephone operator or just simply the operator, PBXs have evolved into VoIP centers that are hosted by the operators or even manufacturers.
A private branch exchange (PBX) in a business usually has an attendant console for the operator, or an auto-attendant, which bypasses the operator entirely.

Digital Private Network Signalling System

DPNSS
The Digital Private Network Signalling System (DPNSS) is a network protocol used on digital trunk lines for connecting to PABX.

Night service (telephony)

Night service
Night service in telephony is a feature of private branch exchanges and other business telephone systems, whereby for a set period during the day (usually those hours outside of normal office or work hours, when normal operator services are not provided), all incoming calls are automatically redirected by the switchboard to a specific extension or to equipment such as an answering machine or other voice mail system.

AT&T Merlin

MERLINAmerican Bell Merlin
One of the most recognized such systems is the AT&T Merlin.
The 1A2 Key Telephone System and later ComKey series (4-16, 7-18, 14-34) had the following problems which the Merlin System sought to solve:

Signalling System No. 7

SS7SS7 networkCCS7
For the option to call from IP network to the circuit-switched PSTN (SS7/ISUP), the hosted solutions include interconnecting media gateways.
This is the case for earlier analogue trunks, multi-frequency (MF) and R2 digital trunks, and DSS1/DASS PBX trunks.

Automated attendant

automated attendantauto attendantauto-attendant
Typically the auto attendant is included in a business's phone system such as a PBX, but some services allow businesses to use an AA without such a system.

Digital Enhanced Cordless Telecommunications

DECTWDECTdigital cordless phones
However, its most popular application is single-cell cordless phones connected to traditional analog telephone, primarily in home and small office systems, though gateways with multi-cell DECT and/or DECT repeaters are also available in many private branch exchange (PBX) systems for medium and large businesses produced by Panasonic, Mitel, Gigaset, Snom, BT Business, RTX Telecom, and Spectralink.

Public switched telephone network

public switched telephone networkPSTNtelephone network
For the option to call from IP network to the circuit-switched PSTN (SS7/ISUP), the hosted solutions include interconnecting media gateways. Early electronic key systems used dedicated handsets which displayed and allowed access to all connected PSTN lines and stations.
There are also private networks run by large companies which are linked to the PSTN only through limited gateways, such as a large private branch exchange (PBX).