Well, thank goodness she's in a nursing home, and your contact will be limited. Sick or not, you have a right not to be abused. Turning off the phone is absolutely acceptable.
I would say to spend as little time as necessary with her. You needn't feel guilt, though I understand it. So, maybe it IS a good idea that you contact a counselor and learn how to set your boundaries without guilt.
I'm sorry you are dealing with this. I'm sending prayers for you and for her...
Shalom!