If your paying your T-mobile bills, you want to use the T-mobile settings.
Originally Posted by bbfaq
TMOBILE USA Users (Success!)
+cgdcont=1,"IP","wap.voicestream.com"
This is a newer string from TMO that works: +cgdcont=1,,"wap.voicestream.com"
Another alternate string (required for BlackBerry 8100 Pearl & BlackBerry 8800): +cgdcont=1,"IP","wap.voicestream.com"
...
If you use T-Mobile:
.........Username: (Your TMobile 10-digit phone number)
.........Password: (Your existing TMobile.com Password)
.........Note: In USA, you can leave both blank