dolphin,
The amount of logic and programming required to add this is too much to just put into a single post; getting the trunk into the system isn't the only hurdle here. It would be best if you have them contact a certified technician who knows what questions to ask and how to program the system to do what they want.
If you were a certified technician who understood telephony and the phone system it would be easier to answer the simple, if any, questions that would pop up.
Sorry,
TE