It's easy. If your not salary, or not on call, or your contract does not specifically state that you need to answer your phone outside of work hours. Then don't fucking respond to anyone from work. Period. Don't make excuses, don't say sorry, don't say your busy, don't respond at all. In fact put all work numbers under one group in your phone and set it to do not disturb to all hours that your not scheduled to work. If your ever questioned at work, just say I plan my week according to the posted schedule and I am unavailable outside of working hours. Leave it at that.