Centrex is a portmanteau of central exchange, a kind of telephone exchange.

Business telephone system

PBXprivate branch exchangekey telephone system
Centrex is a PBX-like service providing switching at the central office instead of at the customer's premises.
From the 1960s a simulated PBX known as Centrex provided similar features from the central telephone exchange.

Extension (telephone)

extensionextensionstelephone extension
Instead, telephone extensions, called Centrex lines, are delivered directly from the local exchange to the user.
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.

Attendant console

attendant console
The operator or "attendant" had a large and complex attendant console, replacing the old, larger and even more complex switchboard and giving additional control of a connection.
An attendant console is a telephone station that is generally part of a private branch exchange (PBX) or Centrex or other private telephone system.

Line hunting

hunt grouphunt groupsline hunting
Stations may also be part of a hunt group, allowing for automatic distribution of incoming calls to two or more extensions.
Also, some phone companies will provide this feature for a small fee (see also: Centrex).

Number Five Crossbar Switching System

5XB switchNo. 5 Crossbar switch5XB
The redesigned equipment was so expensive that usually a separate 5XB switch was used just for Centrex customers, while POTS (Plain Old Telephone Service) customers were wired to an unmodified exchange.
Each vertical group is divided into five vertical files of ten lines, important because class of service, or customer group identification in later Centrex offices, is shared by all ten lines in the vertical file.

Plain old telephone service

POTSplain old telephone serviceLocal Telephone Service
The redesigned equipment was so expensive that usually a separate 5XB switch was used just for Centrex customers, while POTS (Plain Old Telephone Service) customers were wired to an unmodified exchange.
The services include voicemail, Caller ID, call waiting, speed dialing, conference calls (three-way calling), enhanced 911, and Centrex services.

Marker (telecommunications)

Much equipment had to be redesigned, including incoming trunks and markers.
This practice evolved into Customer Groups, allowing the addition of Centrex features to the No. 5 Crossbar switch.

Hook flash

hook flashflashesswitchhook flash
The feature was implemented in Centrex with a timing circuit to detect this hook flash and set up a three way connection among outside party, inside party and operator.
Centrex telephones added a hook-flash button in the 1960s, after some users incorrectly attempted the attendant-recall function.

Last-call return

last-call return*691471
Last-call return, automatic recall, or (on PBX and centrex service) camp-on, is a telecommunication feature offered by telephony service providers to subscribers to provide the subscriber with the telephone number, and sometimes the time, of the last caller.

CLLI code

Entities within client-owned buildings (such as Centrex installations) are coded in the same format as telephone company buildings.

War dialing

war dialingwardialingwar dialer
If a human or answering machine answers, the wardialer program hangs up. Depending on the time of day, wardialing 10,000 numbers in a given area code might annoy dozens or hundreds of people, some who attempt and fail to answer a phone in two rings, and some who succeed, only to hear the wardialing modem's carrier tone and hang up. The repeated incoming calls are especially annoying to businesses that have many consecutively numbered lines in the exchange, such as used with a Centrex telephone system.


remote switching centreDMS-100DMS-250
It provides Plain Old Telephone Service (POTS), mobility management for cellular phone systems, sophisticated business services such as Automatic Call Distribution (ACD), Integrated Services Digital Network (ISDN), and Meridian Digital Centrex (MDC), formerly called Integrated Business Network (IBN).


NFON operates IP-Centrex-telephone systems for businesses and is the only provider of cloud telephony that offers this service specifically for large businesses with 1-249,000 extensions.


Cirpack is an NGN (Next Generation Network) & SBC (Session Border Controller) (http://www.cirpack.com/en) solutions vendor for Telecommunications Operators, Internet and Application Service Providers focusing on telephony services such as residential and business VoIP, IP Centrex, SIP Trunking, Triple play, Fixed Mobile Convergence, LCR, Transcoding etc.

Number pooling

number pooling
The change also caused problems for some office private branch exchange systems, creating an opportunity readily exploited by vendors to sell businesses costly new equipment or hosted Centrex service.


It pioneered the use of "Centrex ISDN" which is a 24x7 dedicated flat-rate ISDN line.

Winnipeg Tribune

Winnipeg Daily TribuneWinnipeg Evening Tribune
On Sunday, June 21, 1970 a new Centrex telephone system was installed for advertisers and subscribers to use when calling.

Via Net Loss

Government subscribers include cities, counties, state, and federal agencies and often included Centrex service.

Junction box

Class of service

class of serviceclasses of service
As related to legacy telephone systems, COS is often used to define the permissions an extension will have on a PBX or Centrex.

Telephone VoIP Adapter

digital telephone adapter
A Centrex TVA VoIP enables centrex phones, such as Nortel P-Phones, and analog-based Centrex phones like BT Featureline.