I don't thinks this solution would apply for your situation, but another way to change is on the trunk group form itself. On the first page, toward the bottom, is a field called Incoming Calling Number - Delete:____ Insert:____ Format:_____
I have a trunk group to my voicemail system and I wanted all outgoing calls from voicemail (paging, call transfer, etc.) to show the pilot number of our voicemail system. An outgoing call from voicemail is an incoming call to the trunk group, so this is where I defined that number.
Since it sounds like you have an outbound call from the trunk group's perspecitve, change pub is the right spot to change.