I am a person who stutters (PWS) and have managed people in the workplace that stutter. PWS can face different levels of difficulty and can be at various places in their journey. Therefore, there is not really “one size fits all” advice. But I’ll try. It appears that the goal you have for him is to be less anxious and more confident. This is a good goal and may result in more fluency as well. One of my employees stuttered a lot at a major Program Design Review. It did not go well. He crashed pretty hard. His lead came to me for advice. I told him that we need to talk openly with him and that he was probably more resilient then you think. We came up with a plan for him to present status at weekly staff meetings. This was a more reasonable challenge and he flourished. Success builds on success. One day, he will do well at the larger meetings. For your specific situation, your employee should get a lot more experience using the phone in low stress situations. To this day, I answer phone calls that are probably spam calls. This allows me to practice, “Hello this is John”. Now that I own that phrase, I am never going back. I have called most of the liquor stores in my area and asked questions. Phone calls for work are easier because I have spent so much time on the phone.