SIP Trunk

A SIP trunk is a virtual phone line that connects VICIdial’s Asterisk PBX to the public switched telephone network (PSTN) using the SIP (Session Initiation Protocol) signaling standard. Unlike traditional PRI or T1/E1 circuits that require physical hardware, SIP trunks operate over standard internet or private IP connections, providing flexible, scalable telephony connectivity. A single SIP trunk can carry hundreds of concurrent calls, limited only by bandwidth and carrier provisioning.

How It Works in VICIdial

SIP trunks are configured in VICIdial by creating carrier entries that define the SIP connection parameters to each provider. The configuration includes the carrier’s SIP server address (IP or hostname), authentication credentials (username/password or IP-based auth), supported codecs (typically G.711 and G.729), maximum concurrent channels, and DTMF signaling mode.

When VICIdial places an outbound call, Asterisk sends a SIP INVITE message to the carrier over the trunk. The carrier authenticates the request, routes the call through its network to the PSTN, and returns signaling responses (ringing, busy, answer) back to VICIdial. Voice audio flows as RTP packets between VICIdial and the carrier. For inbound calls, the process reverses — the carrier sends a SIP INVITE to VICIdial with the DNIS and ANI information.

VICIdial supports multiple SIP trunks simultaneously, enabling failover routing, load distribution, and least-cost routing between carriers. The dial prefix setting controls which trunk is used for specific call types, and Asterisk’s dialplan can implement complex routing logic based on destination number, time of day, or carrier availability.

Why It Matters

SIP trunks have largely replaced PRI and T1/E1 lines as the standard carrier connection for VICIdial call centers. The advantages are significant: no physical hardware required, instant scalability (add channels in minutes rather than weeks for PRI provisioning), lower per-minute rates (typically 40-70% less than traditional circuits), geographic flexibility (connect to carriers anywhere over the internet), and easy multi-carrier redundancy.

For VICIdial operations, SIP trunks enable rapid scaling — spinning up a new campaign with 100 concurrent call channels requires only a carrier account and configuration, not hardware installation. The critical consideration is network quality: SIP trunks depend on stable, low-latency internet connectivity. Ensure your VICIdial hosting has adequate bandwidth, QoS policies for voice traffic, and redundant internet connections to maintain reliable call quality across all trunks. See the VICIdial setup guide for SIP trunk configuration best practices.