It's been three years since I was on call, but ours (280-bed county facility) was:
A minimum rate for carrying the pager (for example, $3.50 an hour)
Two hours' pay for getting called in every time you got called in (even if you were paged while you were on your way in, and the case got cancelled)
Time and a half for the first four hours, from the time you arrived at the hospital to the time you left after the case.
Double-time for any time worked over and above the first four hours.
Hope that helps. I like the phrase "fair and equitable."