How do I break that wall of shyness?

You have to call him back. Guys go through the same thing when they call girls that they really like. I know I get really nervous when I call the girl I like. I tell her to call me back but she never does either, and to be honest it makes me feel like she is not interested. It really hurts if you like someone and they don't reciprocate. If you like someone you have to let them know it EVEN if you are a girl. Guys like girls that much more when they know that a girls likes them.

You have to overcome your fears and 'shyness'. It sounds like you just have butterflies in your stomach, if you want to be with him your going to have to be more comfortable talking to him whether it's on the phone, internet, or real life. Answer his phone calls or even call him. Your going to be really nervous at first but you will get used to it,once you take this big step you'll thank yourself in the future.Just try small talk at first like everyday things and then move onto more complicated subjects.

Good luck,I hope you get what your looking for.

Lol I'm going to give you a "duh" answer, I'm sorry. Next time he calls, just pick up, just do it. Don't even think about it. Pick up and say hi. There probably isn't really a way to get over being shy, you just have to pick up or call anyway. I know you can do it.
Good luck.