I currently do apartment maintenance. We have a regular rotation ,like everyone else I would imagin.There's 11 of us,so every eleven weeks its your turn. However,we can pass it off to someone else if they want to take it.(we get 8hrs pay just for being on call and then paid for whatever calls come in). There are like 3 or 4 guys who are looking for all the OT they can get their hands on and a few others who want no part of it. Management doesn't care as long as there is someone responding to the calls. I dont know if this is done in the HVAC field also.
Occaisionally I'll cover for someone for a few hours or for the night if they have plans,but other than that,I haven't been on nightcall for years.(every Sunday during the day,though. Had to kiss Pats season tickets goodbye,right before they built the new stadium too)