The phone feature won't turn on!

Hi My treo 680 pda feature works great but since 3 days i have been fighting with the phone feature to turn on ands stay on. I did soft and hard resets, got a new sim card ( I have an unlocked one and use it with T-mobile) sometime it goes right into network search and works ok for a couple of hours, then it freezez on a phone call. when I soft reset it it doesn't do the network search and just stays off. by pressing the on button it shows the "hello" but continues to say phone off. On guy from palm support said today i need a 3G simcard for it to work properly, but t mobile doesn't offer that yet. Also it worked fine for a month with the regular sim card. Any ideas?