Bad sound quality of calls through GSM provider

I've set up a VS-GW1600 gateway for routing my SOHO calls through a GSM carrier.
unfortunately i got such a bad sound quality for the external calls, altough my internal calls quality is pretty good.
So I'll like to know how i can manage to enhance the sound quality

I had checked it. It's the customer's cell phone problem. When cutomer calls to my cell phone. It works well. No echo at all.