I think writing a tiny plan, some key points of what you want to tell during the conversation before a phone call would be a nice idea. In this case, you would have all that you want to say in front of you and it would make yourself even a little bit more confident. At least, you wouldn't forget anything.

As for myself, I have this phone fear too. Well, I tend to avoid at any costs. I often forget things (therefore, I like making plans beforehand). I sound submissive. I am afraid. Etc. To overcome this, I got myself a job at the call centre. I'd been working there for half a year. Well, I didn't fully overcome this fear. I'm slightly better at this, I admit. However, when I need to make a call now and I'm afraid as I used to, I tell myself "come on, dudette, you've been working at the call centre. You're good. You can do it" And sometimes that helps