If you go to most hospitals at 2AM and need a specialist of some kind (say a specific type of surgeon), there’s going to be someone in that specialty on call whose going to get paged to wake up, come in, and see you.

Even in family practice, it’s not uncommon to be able to get a call back from the on call doctor at the practice on weekends or off hours — if you’ve got a situation that maybe doesn’t warrant the ER, but you’re not sure if it can wait until Monday.