Hello, ok I have been there, still am, but it's getting better. Practice is key I guess. What has worked for me to calm down is mostly make a scheme of the things I have to say and if it's someone who really makes me nervous even write down the conversation from my side and practice a little before calling. I know it sounds awkward and for the longest time I thought I was really stupid for doing that, but the truth is that if I don't, it'll be a mess. As for answering a phonecall, I have to remind myself that silent is ok. Idk if you are like me, but at the sign of a bit of silence, I start blabbering and laughing and acting just embarrassing, especially with people I know. Then I hang up and I'm facepalming and telling myself, I should have let the silence linger a little so I could gather my thoughts straight and talk like a normal person. I know it is hard, but practice helps the most.