I work for a business that revolves around thier phone system. The system we use is a Definity G3 v12. I have always been under the impression that an outgoing trunk can only have one number assigned to it. So if you wanted to have your actual number to show up on someones caller id you would have to have your own pots line. Otherwise the number that would show up would just be the number assigned to the trunk, which would basically point to the main number for the building. So has anyone ever heard of setting up a trunk through the local phone company that has multiple local numbers assigned to it? Then when the user dials out of the building the PBX outpulses that users extension so that thier actual number shows up on caller id. This does not seem possible to me, but who knows. My boss swears up and down that his last business did this. I told him they probably just had a pots line assigned to everyone, but I could be wrong.
If you have POTS lines connected you are correct. If you have ISDN (digital lines)connected they may be configured to pass the extension number to the central office/parent exchange. It requires the telco to cooperate too.