Infrastructure
Understanding SIP Trunking
How SIP trunks replace physical phone lines, what sits on each side of the connection, and the decisions that matter when you migrate.
12 May 2026 · 7 min read
From physical lines to sessions
A traditional phone system reserved a physical circuit for every simultaneous call. Capacity was bought in fixed blocks and changing it meant changing hardware. SIP trunking replaces that circuit with an internet connection carrying signalling and media between your phone system and a voice network.
The practical consequence is that capacity becomes a configuration value rather than a construction project. Concurrent call limits, number assignments and routing rules can all change in software.
What sits on each side
On your side is a PBX, session border controller or communications application that speaks SIP. On the provider side is a voice network that authenticates your traffic, applies routing policy and hands calls to carriers in the destination country.
- Registration or IP authentication for the trunk
- Codec negotiation, commonly G.711 or Opus
- Number presentation rules per destination
- Concurrent channel limits and overflow behaviour
Planning a migration
Most migrations run in parallel: a small group of numbers is pointed at the new trunk while the rest of the estate continues as before. Once call quality, presentation and routing behave as expected, remaining numbers follow in batches.
Keep a rollback path for every batch. A migration plan that can be reversed within minutes is far easier to approve than one that cannot.