(((((Jane)))))), I'm sorry that your mother can be so difficult to deal with. She sounds like a toxic person and I can see why you wouldn't want to talk to her on the phone.

I think that an alternative to giving people ultimatums is to set boundaries for how you like to be treated and then give that person the option to either respect and honor those boundaries....or they can choose not to.

Remember you cannot control other people, you can only control your reactions to them. Instead of an ultimatum such as "I'm not going to talk to you on the phone until you go to therapy", you could simply say, "I do not have relationships with people who are not actively addressing their life problems." or something similar. Then, at that point, she can decide on her own what sort of changes she needs to make.