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How do you mean? If you want to take your phone abroad and use it the in the US then it will switch to a US carrier, maybe TMobile US, but you will incur roaming charges if you use it.

If you mean can you take a US TMobile sim and put it in your European TMobile iPhone then I would say no, that won't work. You will have to unlock it as it is locked to your country's TMobile.
 
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