You could turn your phone off.
But really, it's up to you to set the precedent. You have to have the will to say "no" when you want. Doesn't have to be "No, I hate you, stop calling me." Could just be "No, I'm doing something with other friends." Be friendly in the classroom, but you are nobody's slave. If it comes down to it, tell her how you feel. If she feels sorry for you, you don't have to make a raw deal. Just know what you want, and say it.
_________________
When I allow it to be
There's no control over me
I have my fears
But they do not have me