Primary Rate Interface

PRI (Primary Rate Interface) is an ISDN (Integrated Services Digital Network) telecommunications standard that provides dedicated digital voice circuits over physical T1 or E1 lines. A PRI circuit delivers 23 voice channels (B-channels) plus one signaling channel (D-channel) over a T1 line in North America, or 30 B-channels plus one D-channel over an E1 line in Europe and most other regions. PRI was the standard carrier connection for call centers before SIP trunks became prevalent.

How It Works in VICIdial

VICIdial connects to PRI circuits through physical telephony interface cards (such as Digium/Sangoma T1/E1 cards) installed in the Asterisk server. The card connects to the carrier’s PRI circuit via an RJ-48C connector and provides a DAHDI (Digium Asterisk Hardware Device Interface) channel driver that Asterisk uses to place and receive calls.

Each PRI circuit supports a fixed number of concurrent calls — 23 for a T1-based PRI and 30 for an E1-based PRI. To handle more concurrent calls, multiple PRI circuits are installed and configured as a trunk group. The D-channel carries call signaling (setup, disconnect, caller ID, dialed number) using the ISDN Q.931 protocol, while the B-channels carry the voice audio in G.711 format at 64 kbps per channel.

VICIdial treats PRI channels similarly to SIP trunk channels from a campaign and routing perspective. Outbound calls are placed through available PRI channels based on the carrier configuration and dial prefix routing. Inbound calls arrive on specific DID numbers provisioned on the PRI and are routed through VICIdial’s standard DNIS-based inbound routing.

Why It Matters

While SIP trunks have become the dominant carrier connection for VICIdial, PRI circuits still have specific advantages in certain scenarios. PRI provides dedicated, guaranteed bandwidth that is not affected by internet congestion — each call gets a dedicated 64 kbps circuit regardless of other network traffic. This makes PRI more reliable in locations with poor or unreliable internet connectivity.

PRI is also required in some regulatory environments and by certain carriers that have not fully migrated to SIP. Some call centers maintain PRI circuits as failover connections — if the internet connection fails and SIP trunks go down, calls continue flowing over the PRI.

However, for most modern VICIdial deployments, SIP trunks offer superior flexibility, lower cost, and easier scalability. PRI requires physical hardware installation, is limited to fixed channel counts (scaling requires ordering additional circuits with lead times of weeks), and typically costs more per minute than SIP. Unless your environment specifically requires PRI for reliability or regulatory reasons, SIP trunks are the recommended connection type.