At my hospital, we have two types of on call time...
If you come in and get regular pay, then yes, once you hit 40 hours a week, you get paid overtime.
BUT, if you come in on call and receive what's called premium pay, where you get time and a half, then NO, it does NOT count toward your 40 hours, nor does it count towards benefits accrual (PTO, etc).
The reasoning is this: the premium pay scale is a different category of hours, by designation. It is a guarantee that you do not get called off later in the week, because you aren't in overtime. If I come in extra, messing up my life to do so, and then later in the week during my regularly scheduled hours they cancel me, what is the incentive for me to sign up for extra work?
The premium pay is expensive, though, so most managers won't offer it. If they call me and ask me to come in extra, I always ask "Are you offering me premium pay?" If they say no, I say "Sorry. Call me back if you are desperate enough, and I'll come in." I usually receive a call back.