One person from each class is on back up call for their class during non-busy off service months where getting pulled away has no negative impact on the rotation you are on. There is no formal payback system, though my particular class has never abused the system and routinely works as a team to try and cover people ahead of time for any reason. No one has ever actually been not sick when they call out. (For me, called in a couple times, one was for a pregnant resident who showed up two hours early because she had been consistently vomiting, tried to sneakily get zofran and a few liters IV before her shift. PD walked by, saw her, and said you're out for this one. Other time was a fellow resident had to be admitted for 2 nights...hard to fake that one.)
Other classes have been thought to abuse the system to some degree though. A payback system is smart, but in my class' case, we haven't needed one (or people will offer to payback anyway).