OK. I've been there. My mother used to do this, ring and ring and ring until I answered. I know what it's like and how horrible and stressful it is when you check the phone and there's 17 missed calls. She doesn't do it any more (TBH she doesn't ring at all, because I don't answer the phone). I will hold up my hand and say I used to do it too because I thought it was normal until my DH told me to cut it out.
I hope it's OK for me to suggest a couple of things.
How often do you want to speak to her, and when would be convenient for you to do it? That's your boundary. You can make it as often or not as you like. Once a year if that works for you. You are under no obligation to answer the phone at any other time. I know it feels like you are, but you aren't. There's no law that says you have to do it and no-one will die if you don't. Boundaries are allowed. It's OK and normal to have them.
The main problem isn't the ringing, it's how the ringing makes you feel. You're answering the phone not because you want to speak to her, but to calm your anxiety and the fact that the ring activates your nervous system in a way that feels very unpleasant.
At some point, to sort this problem, you are going to have to face that anxiety and put in a boundary about when you will answer the phone. There are two possibilities here. You either tell her bluntly to stop, or you ignore. You may have to do both, only you will know that.
I know how hard this is. But here's the thing; if you ride it out, the anxiety will fade. The feeling won't last forever. The body can't sustain it. When nothing bad happens when you don't answer, things will eventually calm. I promise. But the only way to make this happen is to let the anxiety wear itself out. You've got to let it happen.
I would recommend having a plan in advance. She's going to ring, you're going to reply with a message saying I am busy today, I will speak to you on X day. And then you're going to let her ring. If it helps to put the phone on silent (if you can) then do that. Would you be able to get your DH on board, too, so he knows to ignore her too? It will really help if he refuses to be a flying monkey.