First of all, your provider must support changing caller ID to something other than a number assigned by them. Many don't, and in fact, it is forbidden in a number of countries. If they do allow this, you can try putting a number in the Caller Id to be displayed on outside calls field, in one of the extensions, to see if that works. If the provider does not like the number however, the call may fail. There are also some outbound options in the trunk, that can be changed depending on how the provider wants the number to be sent.
I would check/confirm that they allow CID spoofing before you spend time on this.