Hi stpsycho. I have this problem to, but I don't know if I have also social anxiety. Unfortunately the only suggestion that I can give is to try to call despite the anxiety...I know, it isn't a pleasant idea...but recently I had necessarly to call somewhere several times, and I was terrorized, but after a couple times it already became less scary, though it still was difficult...then, slowly, it almost wasn't scary anymore...almost But it's already something
Oh, and using relaxation music before helped to, and also planning and repeating to yourself what you have to say.