How to keep yourself calm during phonecalls?

Rehearse what you want to say, maybe write it down.  Struggling for words often comes from not knowing exactly what you want to say.

During the call, breathe deeply.  Before you start each sentence, take a breath.  If you are struggling for a word, stop trying and take a breath - it will come.

Tell whoever you are talking to that you find phone calls difficult and to make some allowances for you.

There's lots of evidence from laboratory and clinical studies that breathing slowly is a fast and effective way to calm down. Soldiers say that they are able to calm down immediately in dangerous situations this way. Two psychiatrists, Brown and Gerbarg, say a 10 or 20 min slow breathing exercise - 5 breaths a min - is good and 20 min twice a day is a therapy for anxiety. The exercise is inhale and exhale gently, 6 seconds each. You can use the exercise before making a call and slow your breathing during a call. Therapies for stuttering often recommend relaxation. People here often recommend going to StutteringFoundation.org for advice. 

Give short answers. Let the person on the other line pry information out of you