Discover the most frequently asked Phone system questions

What Primary Rate Interface?

Simply put, Primary Rate Interface (PRI) is a way of connecting one enterprise phone system to the public switch telephone network, that is, the outside world. Together with other standards such as Sip trunking, Primary Rate Interface are supported by private branch exchange systems to connect the enterprise network to the global network.


In some countries, or by some providers, PRI lines may be called E1 or T1 lines.

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Technically speaking, Primary Rate Interface is a physical connection which uses circuit-switched models and a dedicated line which only carries voice. By comparison, Sip trunking utilises packet-switched technology over a virtual connection shared with data. However, data can also be transmitted over PRI lines.


Primary Rate Interface may carry 23 voice channels, meaning a company can hold 23 calls simultaneously over one single line. Direct inward dialling, caller ID are supported. One major advantage of PRI lines is that they are end-to-end digital. Quality of service is guaranteed, which is nearly never the case with Sip trunking, although the latter offers many more simultaneous calls. Also, PRI lines are more difficult to tap into and take less time establishing calls than analogue trunk lines.

Disadvantages or Primary Rate Interface lines include high costs for PRI cards, expensive long distance calling rates, and the fact that Inter branch communication is not free.


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