A virtual connection over IP between a customer's phone system or a partner network and a voice provider, used to carry SIP calls in place of traditional circuits.
A SIP trunk replaces physical voice circuits such as PRIs with a logical connection over the internet or a private network. The customer's PBX, UCaaS platform, or contact center sends and receives calls through the provider, which connects them to the public telephone network. For many businesses, it is the main or only path for external calling, which makes availability and security equally important.
Trunks are typically authenticated either by registration with a username and password or by IP address allowlisting, and they are usually configured with limits on concurrent calls, calls per second, and permitted destinations. The same model is used between carriers for wholesale interconnection. IP-based authentication is generally harder to abuse than registration with credentials alone, though both approaches benefit from strict limits and monitoring.
From a compliance perspective, each trunk represents a customer whose traffic the provider originates or carries. That makes the trunk the natural unit for attestation decisions, monitoring, and enforcement action when something goes wrong. Keeping an up-to-date profile for each trunk, including expected volumes and destinations, makes it much easier to recognize when traffic no longer fits.
Why it matters to carriers
SIP trunks are where most toll fraud and illegal robocall traffic enters a provider's network. Setting sensible defaults for concurrency, rate, and international access, combined with per-trunk monitoring and fast suspension tools, lets you contain an incident at a single customer before it affects your interconnects or your RMD standing.