Not receiving 3G network on iphone4?

I got an iphone 4 on t mobile and i don't receive 3G connections, the 3G is turned on. I restored the iphone i put it on aeroplane mode and everything nothing works. I even rested the network settings. Has anyone got any other ideas? Thank you. Note: sometimes when i reboot the phone the 3G network comes on and then turns off after a few mins.

When it is not connected to 3G service is it connected to Wi-Fi? Turn on Cellular Data and see if it connects. It could also just be a location thing, it may connect elsewhere with a better signal.