DID trunking is inbound-only, so if you installed DID trunks, they couldn't be used to make outbound calls. You would need to have another set of trunks to handle outbound calling. Trunks via a PRI work in both directions on a call-by-call basis, so they can handle all system traffic both into and out of the system. The use of a PRI is the most logical choice.

It sure doesn't make much sense to use DID anymore. It also doesn't make much sense to tear out a PBX and replace it with Cisco, but that's another topic althgether.

Ed Vaughn, MBSWWYPBX