Centrex

{{Short description|Telephone exchange service}}

{{about|the type of telephone exchange|the British police training agency|Centrex (police training agency)|the Gazprom-related group of companies|Centrex Europe Energy & Gas AG}}

{{Update|date=July 2019}}

File:ATTtelephone-large.jpgCentrex 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.{{Cite book|url=https://books.google.com/books?id=yVMHAAAAQBAJ&q=centrex&pg=PA367|title=Understanding Voice Over IP Technology|last=Wittenberg|first=Nicholas|date=2009-02-19|publisher=Cengage Learning|isbn=9781111806613|pages=367|language=en}}

Centrex service was first installed in the early 1960s in New York's financial district by New York Telephone.{{Cite book|url=https://books.google.com/books?id=lcqPeexvyqkC&q=centrex&pg=PA2|title=Centrex Or PBX: The Impact of IP|last1=Abrahams|first1=John R.|last2=Lollo|first2=Mauro|date=2003|publisher=Artech House|isbn=9781580536226|at=Chapter 1, Introduction to Centrex|language=en}}{{Cite web|url=https://themuseumoftelephony.wordpress.com/2016/07/09/telephony-101-centrex/|title=Telephony 101: Centrex|date=2016-07-09|website=The Museum of Telephony|language=en|access-date=2019-01-04}} As of 2003,{{update inline|date=October 2023}} it was estimated that there were 20 million Centrex lines installed worldwide by 20 telephone companies, with the most installations in the United States (15 million), Canada (2 million), and the United Kingdom (1 million). This accounted for approximately 5% of all installed business telephone lines, worldwide.

In terms of user-visible features, Centrex and PBX are similar. Features include:{{Cite book|url=https://books.google.com/books?id=vdbRmykbNZ4C&q=centrex&pg=PT143|title=Telecommunications and Data Communications Handbook|last=Horak|first=Ray|date=2012-11-19|publisher=John Wiley & Sons|isbn=9781118585801|at=Section 3.3.1 (Features)|language=en}}

• Direct inward dialing (DID)
• Automatic routing of calls to obtain lowest cost
• Call pick-up groups
• Call forwarding
• Conference calling
• Automatic call distribution (ACD)
• Call detail recording