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As long as the iPhone is unlocked yes. And only the GSM version.



But you will NOT have 3G speeds. It will default back to EDGE (Really slow). T-Mobile's 3G bands are not compatible with AT&T's. And you will not have visual voicemail either.
 
Yes. You will need an either unlocked or jailbroken iPhone with a UMTS radio (no Verizon iPhone4).

You will only get GSM data speed from the phone

You will need to manually enter the SMS/MMS/data servers on the phone to have full service.

If you are an old school T-mobile user like me, you can enjoy 5GB of data before they throttle you to GSM speeds.... but since you are *already* at GSM speeds, its just like having completely unlimited data.
 
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