Hey there! Sorry to read that you're having trouble with your phone! We're happy to help you try to get to the bottom of this, but we'll need a few more details to get started!
Is this a T-Mobile branded iPhone that you're trying to use while roaming internationally, or with another carrier's SIM card? If you're trying out alternate SIMs in your phone, have you already completed a request to Unlock your mobile wireless device? What specifically is happening when you insert a SIM card from another country?
As Marissa mentioned, it sounds like your device isn't unlocked. If it's a T-Mobile iPhone, you'll need to meet these requirements before a request to unlock your phone can be completed. Marissa also had a few questions that'll help us figure out for sure if the device lock is the reason your phone isn't working with other GSM carriers.
Hey there! Wanted to check in here one more time to see if you'd had a chance to take a look at the original reply to your question. We'd love to hear a few more details if you have the chance! Thanks!