I am assuming you are talking about an upper-middle class job? People who work hourly(non-exempt in America), usually wouldn't be getting a phone call in the middle of the night, from their boss. If you do get those calls, and you are hourly, make sure you mark that down as time worked, and if your boss complains, google the laws. Now if you are talking salaried, they can call you at any time, but unless it is stated that you need to be available, you always can turn off your phone.