What is “PRI”?

PRI: Primary Rate Interface


A PRI is a method used by businesses to connect their PBX (private branch exchange) business phone system to the Public Switched Telephone Network (PSTN). It is one phone line, consisting of two pairs of copper wires terminating on the customer PBX. Although it utilizes one line from the phone service carrier, a PRI line can receive/send up to 23 calls simultaneously!

A PRI is channelized much like a T1, but unlike a T1 that has 24 channels, a PRI has 23 usable channels, and a 24th channel used for signaling only. Each channel in a PRI line provides 64 Kbps for data transmission. With PRI, signaling is used to deliver some important features to a customer; for example, a block of DIDs (Direct Inward Dial), or direct numbers.

The 24th signaling channel can also be used to deliver information about the caller, such as the caller ID, where the call is coming from, and what time of day it is at the call’s point of origin.This caller information can be used to route the call to the appropriate person to handle that particular caller.


A PRI line can connect to both Analog/Mixed EPABX systems and also the newer IP PBX systems. A PRI line can also be used to connect two PBX systems thereby providing 30 channels between them for interoperability. PRI can be a wise choice for businesses who are utilizing Unified Communications (UC) within their organization, as it can represent significant cost savings over individual data and voice lines.

There may be a significant savings, however, depending on business priorities and bandwidth, for a compnay to go with a SIP trunk rather than a PRI. Talk to your local telecom consultant to find out what the options are in your area, and which would be the best fit for your business.