Noise with GSM

I’m using Asterisk in a linux suse 10.1 server and X-Lite softphone in a WinXP workstation.

The problem is:

If I use the G.711 audio codec in the softphone all works fine, but if I try to use the GSM audio codec the call is noisy to the point that is completely impossible to understand.

Does anybody know how can I fix it??