My voice drops out on palm treo 650

I have recently (6 mos) begun having problems with my palm treo 650. When on a call, my voice will drop out - I can hear but cannot be heard. It happens anywhere at anytime. I have had the sim card changed 2x but didn't help. T-mobile has no answer for me. This is a new phone (purchased since I thought my previous 650 might have been the problem). Since it still happens on new phone, don't know what else to do. I've had very good luck with palm phones for a few years and just started having this problem recently. I love my treo and would hate to have to switch to a different phone. Any problems noted re: these palm phones? All help is appreciated -