OK, here's the deal. In order to get privileges at a hospital, you have to go into the call pool (occasionally you can get out of it). When you're on call for your specialty, you certainly get trauma-related things (e.g. NSG gets called for head bleeds, ortho for fractures, plastics for facial injuries, etc). Most people out in private practice would rather NOT get called for this stuff because it screws up your already busy elective schedule and the patients don't tend to be insured.
So yes, you will almost certainly HAVE to take trauma-related call for your specialty of choice, but you won't be the "Trauma surgeon", nor will you be qualified to staff an ED as one. And, I suspect, once you're finished with residency you won't want anything to do with it.