so sorry you are feeling this way. i can relate because in the past this fear has almost paralyzed me. first of all, why don't you write down what you need to say when you call for the internship? even if you have to read it on the phone it will help you not get "tongue tied"...meaning not knowing what to say. even if you fail at first at least you are trying.
as for everyday living with a social phobia...what has worked for me is to practice talking to others in small doses. like say "hello" when you answer the phone. it doesn't have to be big accomplishments at first, it's almost like practicing. as time goes on you will build on these successful interactions. it will help you overcome the fears. just take baby steps at first. good luck to you and i hope what has worked for me will help you too.