Write a new driver to a gsm module

I had installed an Asterisk in my raspberry, and I also bought a gsm shield(SIM900).

well, I intend to make the calls from the asterisk use the shield gsm to be forwarded.

So, I would like any information about where to start.

Will I have to write a new driver to make the asterisk recognize my gsm module?( migre.me/o4CuA ).

Where could I get information and documentation about that?