Central Exchange (or Centrex) is a service provided by many telephone carriers to deliver PBX or Key-like services via the telephone company (or Telco.) Little to no hardware is required at the customer end, unless its special adjuncts for digital telephones for power, etc. Regardless of the hardware, the “brain” lives out in the “cloud” so to speak at the central office. If it’s a government, or a large corporation, rarely are these actually installed on site.
Centrex is basically the “business class” equivalent to telephone service, typically most Centrex installs would provide the nearly nineteen features that you could get from your telco provider (call waiting, three way calling, voice mail, caller ID, call forwarding, conference calling, etc.) Many of these features are accessed by calling it’s feature access code or in this case the Custom Local Area Signaling Service or CLASS. Dialing out to get on an outside line was exactly the same as a PBX ether 8 or 9 depending on your dialing plan you had with the phone company. Most telephones are POTS sets or multi line analog telephones you can get at your local office supply store.
Multi appearance terminals have been manufactured, Nortel (Aastra), AT&T/Lucent, Tone Commander and Mitel over the years, and are typically installed in central answering points like the reception desks. This would become supported only in switches that could support ISDN, mostly models made after the late 1980s. The major switches were Nortel’s DMS-100, and AT&T’s 5ESS in the digital sense. Such sets were compatible, but each digital switch had it’s own standards so it wouldn’t be a surprise to see “For use of 5ESS”
Centrex made its appearance for New York Telephone in the late 1960s, as a new product or service to sell to business with multiple lines (that is telephone and trunks.) Despite PBX systems that were on the market (albeit electromechanically), very few actually deployed these similar models used at the central offices (if you couldn’t tell what PBX meant till now…) The PBXes that were installed, were still the manual switchboard; and because automatic dialing had already been established, this would be great selling point from the Phone Company’s perspective. Switching for Centrex services initially began on crossbar and later to early digital switches (I should actually say “digital” in air quotes) made by Western Electric at that time.
Centrex was presumed dead during the Divesture era, with the idea that AT&T would kill Centrex by selling their own systems (like the Merlin for an example.) Because the Divestiture allowed more power for the 7 Regional Bell Operating Companies (RBOCs or “Baby Bells”), this backfired. Also loopholes of the Divestiture enabled the same RBOCs to create shell companies to sell phone systems and IT equipment, even though the original text of the Divestiture prohibited the RBOCs to do so. Centrex as a business model for the local Bell companies grew during this time, and in the recession of the early 1990s, it’s been claimed that Centrex lines was cheaper than PBX lines.
It’s not a surprise to see digital Key phones in a Centrex environment. Because Centrex can’t do paging or intercom in a handsfree way, customers would install a Merlin or Partner (or other brands from other vendors) in a specific department, if intercom, paging or more sophisticated uses would be needed. Also this is why you still would see “Intercom” on these telephones because you’d be making an “intercommunication” call. For the legacy Avaya Red key systems, this was also another reason why trunk access codes were not required. These systems would be installed on top of an existing Centrex (or even a PBX) setup. Personally, I’ve been in places where Partners and Merlin systems were installed and not on top of any PBX or Centrex setup, so the central office always got off hook signals for a few seconds before they would hand off for an inside call.
Centrex had a rise in the 90s bundled with ISDN or T1 circuits, or just more lines to go online with dialup services. But Centrex in name only has become very illrelevent in the 2010s. A decade before names such as IP Centrex attempted to a new buzzword, it was really just a analog to VOIP service. Centrex’s new solution for the 2010s is “Hosted PBX” or “Hosted VOIP” – if anything it’s Centrex with the ability to have multiple lines, access to features with multi appearance sets now can be purchased much less than say an ISDN set in the 90s. Factor the use of “open” *coughs with allergies* standards like *ahhchoo* SIP, users can use any set that can support the alleged open protocol and attach it onto their hosted VOIP system.
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