Here's the thing. To a certain degree we can control when and where we can be contacted. I have a work mobile, provided to me by my employer, that mobile is turned on when I'm at work and turned off at 18:00.
I have a personal mobile but that phone number is not available to my employer as I am not prepared to take work related calls when I am not at work.
I've been asked several times why I don't take calls from work in the evening or at the weekends and in each case my response has simply been that I'm busy and not available for work. Mostly this has been fine, some managers at a higher level who think they are entitled to constant assistance whenever they want have been extremely difficult about this but for the most part the response has just been 'fine'.