That depends. There are three formulas you can use to figure the number of trunks you need to provide. There are the Erlang-B and Erlang-C and then there is the Poisson distribution. The easiest is to look on Erlang tables, or you can buy software. I don’t think mean holding time is a concern when working with Erlangs since that is implied in the unit of measure which defines the intesity of traffic per hour. A person on the phone for a full hour is 1 Erlang; A person on the phone for 15 minutes is .25 Erlang. In order to determine how many trunks you need, you can use one of the formulas listed above, multiply the Erlang per subscriber times the number or subcribers and that will give you the total number of Erlangs that you need to service. Then you go to an Erlang table, you choose a grade or service, and it will tell you how many trunks you need to provide.