Changing your number can definitely cause an escalation. Escalation you don’t need. Do we really ever know how those who aren’t in their right mind will react?
I am curious … your voice mail message is it your voice or one of the prerecorded messages? Either way you could continue to ignore as best as possible she may eventually go away an become fixated on someone or something else. You can also choose to change your number but switch the current one over to some pay by minute phone, or one of the older flip phones that you add on for like 10 bucks a month … toss it out of sight somewhere and charge it every once and while. That will give you peace and also placate whatever it is in her that keeps her calling at this moment.
Court action can been taken as waging a war. You really will have to decide if the war is worth it? If it will really solve the issue. Or if that will just be a way to feed the insanity in her, if that will cause her to escalate. I would ask right from the get go if you do decide on some court action if will have to face her at all, she may enjoy that...