telephony
Extension
An extension is a short internal number that identifies a single phone, agent, or destination inside a PBX so calls can be routed to it.
An extension is the short number that identifies one phone or destination inside a phone system. When someone says dial extension 8368 to reach the sales floor, that number is meaningful only within the PBX — it never travels across the public network. Extensions are how a switch knows which device, agent, or service a call is for once it is already inside.
In a VICIdial system, Asterisk organizes extensions inside the Dialplan. An extension there is not always a person — it can be a number that plays a greeting, sends a call into a queue, or starts a feature. Agent phones get their own extensions too, which is part of how a registered Softphone becomes reachable after Phone registration succeeds.
The point that trips up newcomers is that an extension is internal addressing, not a real phone number. A caller from outside dials a public number, the PBX answers it, and only then does the dialplan use extensions to decide where the call goes next. Most VICIdial operators rarely touch extensions directly because the software builds the dialplan for them, but understanding the idea makes call routing far less mysterious when you do need to read it.
Related terms
Asterisk
Asterisk is the open-source telephony engine that VICIdial runs on, handling the actual placing, bridging, and recording of calls beneath the dialer's logic.
Dialplan
The set of rules in Asterisk that decides what happens to each call, matching the dialed number and sending it to the right place step by step.
PBX
A PBX, or private branch exchange, is the phone switch that routes calls inside an organization and connects them out to the public telephone network.
Phone registration
The process where a softphone or SIP device announces itself to VICIdial so the server knows where to send the agent's calls.
Softphone
A softphone is phone software running on a computer or mobile device that makes and receives calls over the internet instead of using physical desk-phone hardware.