"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. ...
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The PBX operates much as a community switchboard does for a geographic telephone numbering plan and allows multiple lines inside the office to connect without each phone requiring a separate outside line. In these systems, one usually has to dial a number (typically 9 in North America, 0 in Europe) to tell the PBX to connect with an outside landline (also called DDCO, or Direct Dial Central Office) to dial an external number. Within the PBX, the user merely dials the extension number to reach any other user directly. For inbound calls, a switchboard operator or automated attendant may request the number of the desired extension or the call may be completed with direct inbound dialing if outside numbers are assigned to individual extensions." - (en.wikipedia.org 24.10.2019)
