Heard many people having problems using facetime with Tmobile USA.
I know there was a hack on cydia that fixes the problem but your iphone has to be JB.
in my country, there's like 6 mobile phone operators, and
only 3 of them are able to use facetime, the rest operators are like yours, "waiting for activation".
The other poster was right, you don't need 3G to use facetime. The E on your phone stands for Edge, the slower alternative data transfer method. With tmobile the 3G frequency is not used by iPhone. Why not use AT&T?