On site or on call?

ERs tend to be run like concessions and their staff and and hospital staff aren't paid from the same purse. If he's a private physician in a practice, he'd not be at the hospital unless it was his rotation and was called in by the ER attending physician. I don't know how it works in Colorado but that's how it works here. FWIW, I happen to be the both the son and brother of OB/GYNs so I hear a bit about how this sort of stuff works.