Business telephone system

PBXprivate branch exchangekey telephone systemPBXsHosted PBXfollow-mePBXeskey phone systemPBX SystemsPrivate Automatic Branch Exchange
A business telephone system is a multiline telephone system typically used in business environments, encompassing systems ranging from the small key telephone system (KTS) to the large private branch exchange (PBX).wikipedia
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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.

Trading turret

Turrets
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.

Telephone exchange

exchangescentral officeexchange
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.

Caller ID

caller line identificationCLIcall display
Caller ID
A call placed behind a private branch exchange (PBX) has more options.

Extension (telephone)

extensionextensionsextension number
Each device connected to the PBX, such as a telephone, a fax machine, or a computer modem, is referred to as an extension.
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.

IP PBX

IP-PBXIPBXOn-site
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.

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.
It provides functions similar to a PBX, but is provisioned with equipment owned by, and located at, the telephone company premises.

Dial plan

dialing plandialling planopen dialling 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.

Line hunting

hunt grouphunt groupsHunting
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.

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:

Competitive local exchange carrier

CLECcompeting carrierCompetitive 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.

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.

Switchboard operator

telephone operatoroperatoroperators
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.

Public switched telephone network

PSTNtelephone networkpublic telephone network
Early electronic key systems used dedicated handsets which displayed and allowed access to all connected PSTN lines and stations. For the option to call from IP network to the circuit-switched PSTN (SS7/ISUP), the hosted solutions include interconnecting media gateways.
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).

Session Initiation Protocol

SIPSession Initiation Protocol (SIP)SIP Trunking
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.
The service provides routing of telephone calls from a client's private branch exchange (PBX) telephone system to the public switched telephone network (PSTN).

Telephone numbering plan

Area codeCalling codearea codes
In the first numbering plan the PBX maps one-to-one with the numbering plan of the public switched telephone network, e.g. the PBX is assigned all numbers 234-5000 to 234-5999 (1000 devices), and the public switched telephone network treats it as a normal internal switching exchange.
Many organizations have private branch exchange systems which permit dialing the access digit(s) for an outside line (usually 9 or 8), a "1" and finally the local area code and xxx xxxx in areas without overlays.

Telephone switchboard

switchboardswitchboardscentral switchboard
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.

Voice over IP

VoIPvoice over Internet Protocolvoice-over-IP
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. 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 modern key system is usually fully digital, although analog variants persist and some systems implement VOIP services.
In 2008, 80% of all new Private branch exchange (PBX) lines installed internationally were VoIP.

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.

Call parking

park
Call park
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.

Automated attendant

auto attendantauto-attendantphone tree
Auto 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.

Direct inward dial

direct inward dialingDIDdirect inbound dialing
Direct inward dialing (DID)
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.

Voicemail

voice mailvoice-mailvoice messaging
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.

Automatic call distributor

automatic call distributionACDACD splits
Automatic call distributor
Private Branch Exchange (PBX) was a telephone exchange device that acted as a mini-switchboard to route phone calls.

Basic Rate Interface

BRI2B1DBasic Rate (BRI)
Using small PBXs for ISDN is a logical step, since the ISDN basic rate interface provides two logical phone lines (via two ISDN B channels) which can be used in parallel.
The BRI ISDN service is commonly installed for residential or small business service (ISDN PABX) in many countries.