PBX
A PBX, or private branch exchange, is a private business phone system that connects internal extensions and controls how calls move between users, departments, and outside telephone lines.
How a PBX works
A PBX sits between an organization's endpoints and its connection to the public telephone network. An endpoint might be a desk phone, softphone, voicemail box, queue, or application. The PBX applies rules that determine which endpoint should ring, what happens when nobody answers, and whether the call should be transferred, queued, forwarded, or sent to voicemail.
Incoming calls may reach a central business number, an IVR, or a DID number assigned to a particular person or service. Outbound calls pass through the PBX to a carrier connection, often a SIP trunk. Modern PBXs may run as on-site equipment, hosted software, or a cloud service. The placement changes who operates the system, but not its basic routing role.
Why it matters for AI phone calls
An AI phone agent can be connected as another destination in the business phone system. A PBX might send a main-number call to the agent first, route only after-hours calls to it, or receive a transferred call back from it. The agent does not need to replace the PBX; the two systems can handle different layers of the call flow.
That boundary should be explicit during setup. Teams need to decide which system owns business hours, ring groups, voicemail, transfer rules, and fallback behavior. If the same rule is configured in several places, calls can loop, ring the wrong endpoint, or reach voicemail earlier than expected.
A PBX is also not the same as a carrier or a SIP trunk. The PBX controls private call behavior; the carrier connection provides access to external telephone networks. Keeping those roles separate makes an AI phone deployment easier to test and troubleshoot.