Your phone will never choose a carrier who doesn't have a roaming agreement with Tmobile because it wouldn't be able to register with the network (it would show "emergency calls only").

The problem is either in how your account is provisioned for int'l roaming (in T-mobile's systems) or in how you've configured your phone.

Go into Settings -> Wireless & Networks -> Mobile Networks

1. Make sure Data Enabled and Data Roaming are checked.
2. Tap Access Point Names, and then edit your selected APN (should say something like "T-Mobile US")
3. APN Settings are:

• APN: epc.tmobile.com
• Server: *
• MMSC: http://mms.msg.eng.t-mobile.com/mms/wapenc
• APN Type: default,supl,mms
• All other options should be blank ("<not set>")

Also, if you don't have an Android-specific data plan, you might experience problems.

hth, gl