Dear all, I bounght an unlocked N900 in US and used AT&T network for 6 months. It all worked well.

Now I am back to China, using the local network service with new local sim card. Since the bandwidth is supported by N900, it should all work well. But in fact, I can only recieve phone call and sms. I can't call others or sms others. I checked the setting and change it to GSM mode, but still failed.Plus I can not access GPRS of local service
Does any of you have suggestion or experience? Thank you very much.