I just refuse to be a limp dishrag.

Saying "no" to your employer is a pretty tough skill to learn, but it's invaluable. For example, I've said things like "I have a prior commitment that I have to follow through on tonight, and won't be able to stay late. I talked to Joe, and he said that he has other things he can work on until I can get this fixed tomorrow." It's none of my employer's business that my commitment is just going home to have dinner with my wife, or to go play hockey.

Or, perhaps something like "If you're now requiring me to carry a phone, it feels like the job requirements, and your expectations of my duties have changed. If that's the case, we need to review my compensation package, to ensure it's still fair considering these new expectations. If not, then I need to make it clear that outside of my regular work hours, I'll be turning the cell phone off."