Hey! Full time and long time stutterer here! I'm in the same boat, with phone calls causing my stutter to get much more severe. What I've found works, for me, though is this. Take a deep breath right before it's time for the call / meeting, and tell yourself "I'm the baddest mother\*\*\*\*er here." Don't know why, but it tends to work for me at least somewhat. As for continuing to remain calm, you can try several different things, since they can't see you. Fidgeting with your hands or an object sometimes helps. Or since it's an interview, maybe writing down a lot of what they say and ask on a notepad. That way your mind is keeping busy, but you're still paying attention to the actual interviewer and what they say.