:( been there... And most of the time this happens if I'm not even nervous about speaking to the other person, sometimes this happens even if I'm happy/excited to talk to them. It's the pits when it happens. However, something that I found helps me sometimes is to say either Hi! in a loud and lively tone or say the name of the person who's calling you like, "Nelson, Hi!". It works sometimes for me. :-/