PBX

PBX stands for private branch exchange, which is the old name for the phone switch a business owns and runs itself. Instead of every desk having its own line to the phone company, calls flow through one shared switch that routes them internally and connects out to the wider network when needed. Decades ago a PBX was a wall of relays in a closet. Today it is almost always software.

In a VICIdial setup the PBX is Asterisk. It assigns each agent and phone an Extension, it decides where calls go using the Dialplan, and it reaches the outside world over one or more Trunk links to a carrier. Modern PBX traffic is carried over SIP (Session Initiation Protocol) rather than physical copper, so the whole switch is just a program running on a server.

For a newcomer the useful takeaway is that VICIdial is not a phone system on its own — it is a dialing and reporting layer that sits on top of a PBX. The PBX handles the plumbing of connecting two parties; VICIdial handles the strategy of who gets connected, in what order, and with what data on screen. When you hear someone say a call never left the box or got stuck on the way out, they are usually pointing at the PBX layer, not the dialer.