I used to be terrified of the phone. I am 34 with a severe stutter. The first thing you should say is some form of, “I have a speech impediment please bear with me.” I have surprisingly only been met with courtesy at that point and it has a double whammy of lessening the pressure. You will sound terrible the first few times. That’s okay. Keep doing it over and over and the fear and tension lessens and your confidence will rise. I promise you. Exposure therapy fucking works. You won’t be totally fluent but you will be successful. Please be kind to yourself. It’s only a phone.