Can I just say that whatever you do, please don't unplug your phone or change your number, as this would mean that she can't get you if she really needs you in a genuine emergency.
As I think one or two other posters have said, get yourself an answer machine, and tell her that you've got one, so that if you can't get to the phone, ie you're in the bath, or busy doing something, or you are out, she can always get hold of you by leaving a message, and you'll give her a call back as soon as you can. That way, if she's just pestering, you can pretend you're out, and then finally give her a ring when you're good and ready, but if it should be an emergency, you can call her straight back, as you will never forgive yourself, however much she's bullied you over the years, if you switched your phone off and she died while trying to get your help.
The rest of the advice is all good, and you can definitely manage her better by not giving her so much information about your everyday life. You're not too old to learn how to assert yourself, just try taking it one step at a time.