Primary Rate Interface (PRI) is a digital ISDN service that delivers multiple voice channels over a single circuit to connect an enterprise PBX to the public telephone network. If you’re asking what is Primary Rate Interface, it’s the standard that historically powered business voice trunks: 23B+1D on T1 (North America) or 30B+1D on E1 (most regions), with 64-kbps bearer (B) channels for calls and a signaling (D) channel.
We often see PRI in sites running legacy PBXs, regulated environments, or locations with limited IP voice options. It’s valued for predictable quality and stable signaling, but capacity scales in fixed blocks and relies on physical telco circuits.
Key characteristics include:
- Dedicated capacity: Reserved channels ensure consistent call quality.
- DID support: Efficient routing with direct-inward dialing and caller ID.
- Fixed scaling: Add capacity in PRI increments (e.g., another T1/E1).
- Physical dependency: Requires telco handoffs and on-site interfaces.
Our take? PRI is proven and predictable—but many organizations are now moving to SIP trunks for finer-grained scaling, redundancy over IP, and cost flexibility.
Planning a graceful exit from PRI without disrupting voice services? Explore our PRI Replacement Guide for a step-by-step path to SIP trunking—number porting, E911, survivability, and hybrid cutovers included.
