When they call me at home, I don't answer (this is one reason I continue to have a land line). Of course, I am rarely home during my off time anyways so that's kind of a no-brainer. I was up front with my last manager and former team lead. I told them straight... if you want me off hours, you pay me to carry a pager, otherwise, I'm unavailable and you will not get me.
Good strategy. If they have you on call they should give you some kind of remuneration.