In America, if your boss calls you outside work hours, we don't answer the phone and let it go to voice mail. Some people say rude things about our bosses while the phone rings. If your boss wants to do something outside of working hours, they should have made sure that you're available DURING working hours.

You're better off not answering because if your boss is trying to offer you a shift, and you can't make it, then you're looked at as unreliable - on a day that you weren't supposed to work in the first place.

I think it's just a matter of communication between you and your boss. Tell your boss if you'd be interested in extra hours, or picking up shifts when people call out sick. And also tell them when you're no longer available for this. Your time is your time and you deserve it, but if you're willing to help out in a crunch, then you're also a superhero.