After hours of searching for a solution to this problem that I am experiencing, I still have no solution. Tried to reset network settings, all settings, hard reset, factory reset my phone and still no luck. I recently put a 3G sim (chopped it at a shop) into my iPhone(4 unlocked) and I am experiencing this very annoying issue. I have no bars and my carrier is displayed and everything, However I just cannot make a call or text! Does anyone have a solution to this problem? I have ios 5.0.1 and activated phone using iTunes, tried to downgrade but have me an error.. Any help is appreciated thanks.