I think that it's because when you use Call Forwarding Off-Net, the call is now coming from the trunk and not the station. Just a guess.
You could create an x-port station with a remote coverage path, call forward to the x-port, and see what your caller ID is. Since I do not allow Call Forwarding Off-Net, this is one method I use to route calls off my network.
Susan
You can have it all. You just can't have it all at once.