While this usually only applies to part time workers, it can also apply to full time workers when called in to work overtime.
Sometimes it gets so bad that:
* You're afraid to pick up the phone and/or will screen all calls (and will be very thankfull for caller ID)
* You'll be unable to sleep in at all because you cannot relax because they might call.
* You'll be afraid to make plans of any kind because you might get called in. (ie, it's your birthday or a relatives birthday and you have the day off and want to take them out for supper, but are afraid to plan beyond 'if I don't get called in maybe we could' because you cannot count on actually having the day off)
* You cannot relax even in hobbies that used to give you great joy because the worry you're going to get called in.
Depending on the nature of the hours you work, after a certain time durring the day you can probably relax as a call in is unlikley after that point. However, if they can also call in you in to work a night shift, it can continue all day :(.