I’m not sure if it’s law, but at the hospital I work at (a very large medical center), we are not allowed to work beyond 16 hours in a day, although I think on-call is different.
Course management is so focused on the bottom line that many times when it looks like the census will be down, supervisors will start calling nurses and telling them that they don't need to come in for their shift. Of course, there is a chance that things will change, the ER will get more patients than expected that need to be admitted and then the superviors are looking for units that have enough staff. Not enough staff, patients lay around in the ER for hours till the next shift comes in. It can look like one big chess game sometimes.
We don't have "on-call" provisions for bedside nursing but certain departments such as surgery, evening, night and weekend ultrasound and other diagnostic testing, biomed repair work does. The staff for these departments usually average just one on-call day a month so it really doesn't interfere with their lives too, too much.