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Centrex
Business phones
Businesses used to have a single telephone number for the main switchboard of
that business and a switchboard operator would connect the call within the
business. If the called party didn't answer, the caller was typically
transferred back to the switchboard. With voicemail and more technology,
businesses now use Direct Inbound Dialing (DID) (Direct Dialling Inwards (DDI)
in the UK) lines so that outside callers can call to a specific person in a
business. Often, the DID number uses a pattern from the called party's telephone
internal extension. For example, within a corporation, a caller may dial 225 to
reach Mr. Smith, but an outside caller may dial 448-9225 to reach Mr. Smith
(with the last three digits representing Mr. Smith's extension). Some companies
restrict the DDI capability to certain parts of the PABX or Centrex numbering
range: Mr Smith on 225 might have it, but Mr Jones on 374 or Mr Brown on 427
would not. If a merger has happened in the past there may even be different
exchange codes mapping to different parts of the internal numbering range. DID
became an important feature of Centrex.
Some companies have a sufficient need for internal extensions that the whole
numbering range following the exchange code (448 in the example) represents
company extensions. A real example of this in the UK is the former Post Office's
headquarters in Central London: the PABX was made an end node exchange in the
London Director area, with the code 432. The automanual board was given the
extension number 1234, so in the days before all-figure numbering outside
callers reached the switchboard by dialing HEAdquarters 1234. The White House in
Washington, DC is another example, having the (202)456 exchange.
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