I bought a used Treo 600 in USA to use in China. It is suppose to be unlocked, but was used on AT&T network in USA. I have a China Mobile SIM card that works in my Chinese purchased Siemmens phone, but my Treo 600 microphone does not seem to be working... I can send and receive SMS text messages, hear people when they call, but they can not hear me. Am I doing something stupid as a new Treo 600 user? Any suggestions?

Thanks