What Is a Pri Phone Service?

By Naima Manal

PRI phone service is the large-volume phone service of ISDN, or Integrated Services Digital Network. ISDN is responsible for transmitting voice and other data communications in digital form through the same telephone copper wiring of the older analog telephone systems. In order for PRI phone service to work, adapters are located at each end of the communication. The ISDN communication system is used in the United States and in Europe. The phone line connection in the United States uses a T1 system, and the phone line connection in Europe uses an E1 system.


PRI, or Primary Rate Interface, provides telephone service to businesses that require large and complex phone systems over the ISDN system. It's counterpart, BRI, or Basic Rate Interface, is designed for home or small business use.


The PRI phone service has 23 B-channels, or bearer channels, and one D-channel, or delta channel. The delta channel is responsible for the controls and signals for the 23 bearer channels that are used to transmit voice and data information through the lines.

T1 Lines

The lines that are used to carry the PRI phone service are the T1 lines. A T1 line is a phone line connection that can either carry 24 different voice channels if used for phone communication, or tranmit data at 1.544 megabits each second if used for a network router. Therefore, the T1 line can carry all 24 lines of the PRI system--the one D-channel and the 23 B-channels. In Europe, however, the PRI service uses 30 B-channels with the one D-channel, and they use the E1 line--E is for Europe--rather than the T1 line.


PRI phone service with T1 lines can handle the large telephone systems of call center operations and large corporations. The 23 bearer channels replaces the same amount of separate analog phone lines, and each PRI B-channel can be easily assigned to meet the specific tasks of its users, such as for fax machines, Internet and video conferences.

Modem Support

According to John Shepler of T1 Rex's Business Telecom Explainer, the PRI phone service also makes it possible to support different modems through its bearer lines. This will prevent a lot of users from overloading a single modem. Rather, the increase in Internet modem capability will allow many users to have access at the same time.

Cost Effective

PRI phone service is cost effective for many large businesses and operations. It eliminates the need for the company to have multiple telephone plans, because of the number of connections one T1 line can support.