they pretty much don't.. a trunk is a single path to a far end switch, either a local carrier, ld carrier or another pbx.. here's the path.. you tn (set) dials 9, that sends you to ld 90, then dial 1213 ld 90 sees a npa match, that sends you to an rlb, (ld 86) that picks a route (1st idle trunk). that is where the switch checks your ncos, if allowed your tn is soft connected to a trunk tn.. and the correct digits are outpulsed... you can have trunk to trunk connections... in my 81, if i see a inbound number xxx 17xx, i send that call to route 97, a tie to an opt 11, they get all did's 1700 to 1799.. that uses ld 97, cdp/dsc.. when you put a call on hold, the trunk is on a conf with a music trunk..
john poole
bellsouth business
columbia,sc