I am sure anyone who lives in a country where a different language to their mother tongue is spoken will understand straight away how absolutely terrifying speaking on the phone can be. In real life, face to face conversations you have so many things to help you through the fog of foreignness:
You can see some-one's body language and facial expressions; You can gesticulate to make yourself understood; You may be able to have a dictionary to hand and pour over it with the person who you are talking with; You can draw things on a piece of paper.
On the telephone all these things disappear and you are left with just the voice of the person at the other end. No facial expressions, nothing. Actually, not nothing, because there are other things to make telephone conversations even harder.