PBX stands for Private Branch Exchange System, which is a private telephone network used within an organization. The users of the PBX phone system can communicate internally (within their company) and externally (with the outside world), using different communication channels. PBX mainly supported ISDN and analog lines, but today PBX also supports Voice over Internet (VoIP).

Let’s understand the similarities and differences between ISDN PRI and T1 line services on PBX.

What is the T1 trunk line?

A T1 trunk line uses two twisted-pair copper wires to transmit and receive data or voice traffic.  This early form of data connectivity was developed by the Bell System to bring data connectivity to the vast majority of businesses.

It’s easy for one hardware interface to support T1, Channelized T1, and ISDN PRI because they are all variations on the same basic circuit. The circuit for both these services is based on the T1 trunk line specifications.

The service differences from a business perspective lie in how this circuit is configured for particular applications.

How does ISDN PRI work?

Integrated Services Digital Network or ISDN provides Primary Rate Interface or PRI service. It is a high-capacity service carried on T1 trunk lines between telco central offices and your location.

PRI divides a T1 trunk line digital signal into 24 channels of 64 Kbps capacity per channel. Twenty-three of these channels can be assigned as one telephone call each, the equivalent of having 23 separate telephone lines. The 24th channel is used for data such as signaling information and special features such as caller ID and hotel/motel information services

How does the T1 Line service work?

T1 lines can transmit voice and data much faster than a regular telephone line leading to their immense popularity. At a speed of 1.544 Mbps, T1 lines ensure that higher-speed connections are delivered reliably across the enterprise. These can be delivered as unchannelized or channelized services.

Unchannelized T1

Unchannelized T1

Unchannelized  T1 treats all 24 T1 channels as one big combined channel for carrying Internet service, point-to-point data download, or VoIP broadband phone. This service is intended to be handled by digital routers, not PBX telephone systems. There are no dedicated phone channels or signaling assigned for switched telephone service.

Channelized T1 lines

Channelized T1 lines offer bandwidth segmentation across 24 channels where every channel can carry up to 64 kbps. Since there is no separate channel for data, signals and other information are sent over the 24 lines by “robbing” a small amount of capacity from each.

Comparing ISDN PRI and channelized T1 Lines


In channelized T1, businesses typically lease the T1 trunk line itself.  When the customer leases a T1 line, it is dedicated entirely to that customer, ensuring that the bandwidth is stable as well as reliable.

In ISDN PRI, the calling is done over ISDN’s underlying circuits.


PRI is a robust service making it appropriate for businesses requiring multiple voice lines or massive quantities of bandwidth to be sent across the network. But scaling is difficult due to underlying circuit dependencies.

T1 lines can be bonded and scaled for additional bandwidth and speed.

Additional services

Channelized T1 doesn’t provide any capability for ANI or caller ID data. ISDN PRI has a few additional features like caller ID that can be added to the service.


T1 line can prove to be expensive if you don’t utilize the fixed bandwidth that you pay for. You can use fractional T1 to reduce your PBX expenses while still enjoying T1 benefits like speed, reliability, security, flexibility, etc. ISDN PRI connections, on the other hand, cannot be fractionalized or halved to reduce expenses.

Choosing the best option for you

The terms PRI and T1 are often used interchangeably, and this can lead to some confusion. Both ISDN PRI and T1 lines come with their own benefits making them more suitable depending on the situation. It is also important to note that ISDN PRI services are gradually being switched off in the US.

To understand the best telecom options for your business, contact us at CarrierBid Technologies today!

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