SIP trunking

Legacy PRI circuits carried a fixed number of simultaneous calls — 23 or 30 depending on the region — and adding capacity meant provisioning more physical circuits. A SIP trunk carries calls as IP traffic, so capacity becomes a configured limit rather than a hardware constraint, and it can be raised or lowered without a site visit.

Trunks are the enforcement point for several things that matter operationally. The provider applies a concurrent-call ceiling on the trunk, which is a hard cap regardless of what an application attempts. Authentication is usually by IP allowlist or SIP digest credentials. Outbound caller ID presentation is validated at the trunk, and the number a business is permitted to present is bound to what it has proven it controls.

For a voice agent, the trunk determines what happens under pressure. When a dialer exceeds the trunk’s concurrent-call limit, new calls are rejected at the SIP layer rather than queued, so a campaign that ignores its own concurrency ceiling produces a burst of failed calls rather than a slower campaign. Pacing to stay inside the trunk limit is a prerequisite, not an optimisation.