Have you turned off the proxy on your connection settings?
T-Mobile ave left a nasty impression on me.
There's a broken network cell in my area where only GPRS works, so you have to force the phone to connect through that and not 3G (3G connects, but no data will transfer), they refused to believe me, despite testing with a collegues handset and sim proved the same, and walking to the other end of town allowed 3G to work, and back came back again and it didn't, been like that for months. Tech support wanted to blame my settings / handset / anything but their hardware, despite me proving several collegues had the same problem.
I'm just in the process of moving my number to Three (Three and T-mobile are the only choice if you want a large data pack)