The phone is very tricky. In my worst moments I've been like 30 mins with cold sweat looking at the phone without being able to press the call button, and being in the receiving end was like getting mugged lol. So yep, I can painfuly relate to that. If you are expecting a phone call, you may also try to make a small guide of the points you want to talk to keep you focused or even write down the important lines and read it (reading is usually easier than speaking) or any technique that works specifically for you. But all those are tricks anyway, work on your confidence and controling your anxiety. I'll get better with your stutter as a whole, so keep trying and also practice phone calls specifically. That's the thing that'll make you start picking calls as soon as you can reach the phone. Get better soon BTW!