There are laws about having people on call, etc.  For example, where I live there is a law that states that if you are given vacation time they cannot ask you to be on call or ask you to work, you must be considered absent the entire time.  There is also a local law (county law) that states that if you are on call you must be compensated, and that every individual must be given 8 hours between ending one shift and beginning the next... they had some issues where a local tech company was making people work 12 hours on one day and then beginning them immediately the next day for another 12 hours, so 24 hours straight which is illegal.
Anyways, call your local and ask.  You never know.