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Centrex 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.<ref>Template:Cite book</ref>
Centrex service was first installed in the early 1960s in New York's financial district by New York Telephone.<ref name=":0">Template:Cite book</ref><ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> As of 2003,Template:Update inline 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).<ref name=":0" /> This accounted for approximately 5% of all installed business telephone lines, worldwide.<ref name=":0" />
In terms of user-visible features, Centrex and PBX are similar. Features include:<ref>Template:Cite book</ref>
- 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
Phasing outEdit
In the United States, the usage of Centrex lines has fallen from 16.5 million in 2002 to 10.7 million in 2008 as users transition to IP-PBX (through VoIP).<ref name=Reed1>Template:Cite journal</ref><ref name=ict1>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> Centrex continues to be used by large institutions, government agencies, and universities as most of the equipment has already been paid for, though leasing Centrex lines may be more expensive.