iPhone does not support T-Mobile 3G. You can use the Edge of T-mobile on the iPhone though but you have to change the cellular data APN to one of t-mobile inside the network settings of the iphone.
Here are some APN that i know of try one to see which one works. Remember this is EDGE and not 3G. Try the last one first.
T-Mobile Internet: Internet2.voicestream.com
T-Mobile Internet VPN: Internet3.voicestream.com
Web2Go/t-zones: wap.voicestream.com
BlackBerry: blackberry.net
Sidekick: hiptop.t-mobile.com
T-Mobile G1: epc.tmobile.com
Here are some APN that i know of try one to see which one works. Remember this is EDGE and not 3G. Try the last one first.
T-Mobile Internet: Internet2.voicestream.com
T-Mobile Internet VPN: Internet3.voicestream.com
Web2Go/t-zones: wap.voicestream.com
BlackBerry: blackberry.net
Sidekick: hiptop.t-mobile.com
T-Mobile G1: epc.tmobile.com