Only way to overcome that is to expose yourself to speaking on the phone. I was exactly like you all throughout my teens and early 20s. Now I'm 35 and speak on the phone all the time for my job and personal life. I don't even think about it. What I did was started by calling department stores and just asking simple questions. Then I got this app where you just talk to random strangers all over the world. You just need to expose yourself to talking on the phone more and more. You'll realize stuttering doesn't kill you, your anxiety will be less, and you'll actually become more fluent in the process. And you'll see the changes much faster than you could imagine. TLDR, Stop avoiding the phone. Go on the offensive. Stutter your way through ordering food for take out. And then do it again, and again and again. The only way out of this problem is through.