Hi Daniel,

To determine the number of circuits required for an Installation is a bit of an art and does require some known variables to do an accurate calculation. There must be some history or patterns or you are just guessing (which is OK too :). 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 (like you nicely noted).

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.

You can always engage your service provider to do a Traffic Study that will also include busyhour averages in your current setup. It is probably good to be over-trunked rather than under-trunked, especially with an Enterprise environment. You may want to design for Failover in some environments as well.

1 Trunk (in your case channel) per every 10 users, so

100 users = 10 channels

200 users = 20 channels

You can also use RTMT to monitor your PRI's and send you email alerts when defined thresholds have been exceeded.

As you can see this is not an exact science for example;

For our Student Residence we use 5 PRI's for 1000 student phones.

For our Staff we use 4 PRI's for well over 2200 phones.

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

http://personal.telefonica.terra.es/web/vr/erlang/eng/mcerlc.htm

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

Rob