I have the same issue as well, i stutter at calls at well. At work ill stutter less, but i do stutter but when i take calls for work, i stutter a lot. I can put a message to my Co worker, but the problem is he never sees my message, rarely he sees it. Si i am forced to make a call. I am just scared to make one cause i know i stutter. I need to get my fear away. If i get my fear away I'll not stutter. Try to calm yourself, breath and think of what you want to say and then say it.