Congrats on the interest. Ah, the first phone call...very cool. My advice to my son was to write down his questions/comment before hand, and keep it real simple. I knew he'd be a little nervous, and there was no threat of my son talking too much (he's not a big talker but he is a great listener), so I suggested he write his questions down and carry them with him. We rehearsed the call many times, and the goal of the conversation is to keep their interest, continue the dialog, and to request a face to face meeting (camp or showcase) in the future. I tried to make it Sesame Street simple. This worked for us. I had a very shy 16 year old who was not a natural conversationalist with strangers. This helped him tremendously, and he quickly gained a lot of confidence in speaking to coaches. My point is if my son can do it, anybody can do it with just a little practice and a simple formula.
Good luck!