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New Member

pri and calls made

Is it correct that a PRI allows 23 simultaneous calls, and no more than that? Also, is there a good calculator for determining how many PRIs you might need for "x" amount of phones?

Hall of Fame Super Red

Re: pri and calls made

Hi Mike,

You are correct :) In North America we do use T1-PRI which is 23 B-Channels ( simultaneous calls) and 1 D-Channel (Signaling). In Europe and other parts of the world E1-PRI's are used which have 30 B-Channel and 1 D-Channel.

To determine the number of circuits required for an Installation is a bit of an art and does require some known variables to do a calculation. There must be some history or patterns or you are just guessing. These numbers are not standard from deployment to deployment and depend on things like if there are Call Centres, Outbound Sales Campaigns,acceptable busy signals etc. etc.

The Telco's use a rule called the "1 in 10 Trunking Rule" which says that for every 10 phones 1 will be in use at any given time. In a Campus environment like ours, if you take the Call Centres out of the equation the actual use would be more like 1 in 15 or 1 in 20. So you Trunk accordingly.

If you know any of the variables you can use a calculator like the one below to help determine the number of circuits required.

I hope this helps, there is no way to give you an exact answer here :)


Cisco Employee

Re: pri and calls made

It's true that a T1 PRI has 23 bearer channels and that you could only have a maximum of 23 simultaneous calls going through it.

With your partner level access you may have access to some tools on CCO here:

For more reading on traffic analysis for the purpose of sizing resources, see also:



New Member

Re: pri and calls made

Thanks to both of you.