I think the fact that your boss makes you feel uncomfortable or less than for something you don't have control over is pretty sad. Does he not view stuttering as a speech disorder. I would schedule a formal meeting with him, and tell him about your stuttering. Explain that it's a speech impairement and that's something you gave little control over. Explain to him that you need support at your job. Go over ways how you'd like to be supported at your job, whether that be less phone time and more work that doesn't involve phone calls. Or if he'll allow you to use your phone to pre-record frequently said phrases and intros for when you do calls. Also tell him your history with it, ie how long you've had it and if there's any family history and also your daily struggles. This will help him see the scope of the disability and how it's affected you.