Architecture

SIP Trunk

A SIP trunk is the carrier-side endpoint that connects your PBX (or AI voice agent, or softphone) to the PSTN over SIP. It is the modern replacement for the analog or T1/E1 'trunk' that PBXs used to have, and it is how every cloud-native telecom service reaches the phone network.

What a SIP trunk provides

• Outbound calling to any E.164 destination, billed per minute.
• Inbound calling on one or more DIDs assigned to the trunk.
• SIP signaling endpoint (SIP server FQDN + port) and authentication mode (digest or IP-ACL).
• Concurrent-call capacity (channels), usually metered or capped per plan.

Authentication modes

• SIP digest auth: username + password. Works behind any NAT. Required for dynamic IPs.
• IP-ACL: the customer's static public IP is pinned. No registration overhead. Recommended for cloud / static deployments.

Routing model

1. Outbound: PBX places call → SIP trunk → carrier's PSTN gateway → destination.
2. Inbound: caller dials your DID → PSTN → carrier → SIP trunk → PBX → extension/IVR/agent.

Related terms