How to get rid of people who only call to make you sad?

I would suggest that you stop answering the phone, but since she seems to create issues with that, I would suggest picking up but simply saying that you are just oo busy and don't want to be rude but half to go. You don't have to tell her you don't like talking to her, but involving yourselfmore in your life and trying to avoid having such long and depressing discussions with her will make you feel better and make it clear that your interest in talking to her is waning fast.