Last Updated on January 28, 2026

Primary Rate Interface (PRI) is a technology used for telecommunications on an Integrated Services Digital Network (ISDN) to transfer digital voice and data between users and the network. PRI technology is integrated with ISDN and grew in popularity due to its ability to scale business communications. This article will define PRI, describe its advantages and disadvantages, and discuss how to integrate call recording into a PRI environment.

What is PRI?

PRI is a telecommunications service standard for ISDN networks. ISDN allows businesses to transmit multiple phone calls as digital audio over regular analog Public Switched Telephone Network (PSTN) lines. Businesses with departments that are required to support high call volumes such as call centers use PRI to facilitate concurrent connections. Learn more about what is an ISDN.

T1 vs E1

PRI is based on a Transmission System 1 (T1) in North America and Japan, E1 in Europe and Australia. The table below compares T1 and E1 systems.

Feature	T1	E1
Location	US, Canada, and Japan	Europe and Australia
B Channel Transmission Rate	64 Kpbs	64 Kpbs
Trunk Line Transmission Rate	1.544 Mbps	2.048 Mbps
Maximum Channels	23 + 1 Delta (24 Total)	30 + 2 Delta (32 Total)
Signaling Channels	1 Delta	2 Delta
Voice + Data Channels	23	30
Material	4 copper wires divided into two pairs	4 copper wires divided into two pairs

How does a PRI work?

PRIs are commonly used by large and medium sized businesses to provide access to the PSTN. A single PRI line provides up to 23 simultaneous connections meaning 23 individual employees may be on a call, having a video chat, sending an Instant Message, or communicating with other individuals outside the organization on a T1. An E1 supports up to 30 simultaneous connections. Each connection is capable of transferring up to 64 Kilobits per second (kbps) of data. Businesses can add additional PRI circuits to scale beyond 23 T1 and 30 E1 connections.

What are the features of PRI?

PRI provides many features to businesses that utilize this technology. Some of the key features of PRI are:

• Automatic Call Distribution (ACD) – ACD is a telephony system that routes incoming calls to the intended recipient. Private Branch Exchange (PBX) systems provide ACD features and the PRI delivers multiple calls from the PSTN to the PBX. Learn what is a PBX
• Direct Inward Dialing (DID) – PRI provides support to DID which is a telephone service that assigns employees a direct number. Calls arrive to the employee directly and bypasses the main line.
• Scalable – Businesses can add multiple PRI circuits to increase beyond 23 T1 and 30 E1 connections as mentioned in How does a PRI work.
• Flexibility – PRI channels can be configured to be dedicated to data, video, or voice. This allows businesses to adjust based on demand.

What are the benefits of PRI?

The primary benefit of PRI is allowing companies to have multiple calls simultaneously. There are additional benefits such as:

• High Quality of Service (QoS) – PRI circuits have dedicated bandwidth and typically provide a higher QoS. Alternatives such as Session Initiation Protocol (SIP) line must support other bandwidth uses.
• Secure – Calls are performed on physical wires. An individual must tap into a physical wire or transfer the call to an internet connection to intercept a call.
• Traceable – Emergency responders have the ability to trace calls to the exact location in a PRI environment.
• One Single Line – PRI allows companies to transmit up to 23 calls simultaneously over a single T1 or 30 calls over a single E1 line.

What are the cons of PRI?

There are a few drawbacks of PRI such as:

• A PRI line is limited to 23 connections on T1 and 30 on E1. Purchasing another PRI circuit is necessary if a business requires additional connections.
• International or long-distance calling may be more costly when compared to alternatives such as Voice over Internet Protocol (VoIP).

How to Record Calls on a PRI environment

Businesses that are focused on improving the performance of their call center may setup call recording which allows managers to monitor, review calls, and provide coaching to improve the performance of their agents. This is why recording calls is important for any business.

Lines must be physically tapped to properly record calls on a PRI environment with either T1 or E1. An effective method is Call Recording Scenario 3 – Business Phone System, Passive Recording From The Trunk Lines. This method allows companies to record multiple telephone calls at the same time, supports several PBX models, and supports both analog and Time Division Multiplexing (TDM) lines. The image above represents how Scenario #3 is setup. Scenario #3 is configured with Passive Recording tapped on the trunk side connected to recording boards. Learn more about Recording Calls on T1 / E1 / ISDN PRI Lines.

PRI and Call Recording

A PRI phone system benefits companies of all sizes. This is especially true for companies with call centers that make and receive multiple and simultaneous calls. PRI lines provide scalability, flexibility, and provides businesses the ability to add or remove telephone lines as needed. PRI also allows multiple telephone lines to be used by one single T1 or E1 line empowering companies to have several telephone numbers for multiple departments.

Companies with PRI infrastructure and are focused on growth will benefit by implementing a powerful call recording software platform like Versadial Solutions. Versadial allows business managers to improve customer service interactions, refine call center scripts, ensure compliance, enable coaching for sales, and provide multiple other benefits. Learn more about Call Recording Benefits and How Versadial can help.

Contact Us for more information about call recording and call center solutions for your business.

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Last Updated on January 28, 2026