Hello Maria. There is a service that can remove the blacklist status of your T-Mobile USA iPhone. Then, you can use it with any network you want, even inside USA. The only "problem" is that IF the mobile network wants to reblock your IMEI number (If they make a routine check), your iPhone will become blacklisted again. Doing an IMEI check first, will make things clearer, since we will learn the reason of the blacklist status (Which is important!).

I suggest you first check the status of your iphone in unlockspector, then unlockspector will suggest next steps for your iphone blacklist

Probably not. I would not recommend it.

It didn't work for me