SIM card for iPhone not supported?

I recently bought a T-Mobile iPhone through Gazelle's ebay store. I went into the T-Mobile store and gt a sim and went home to activate one of the prepaid plans. When I went to activate my phone all that comes up in Sim Not Valid. It says that it is not supported under activation policy. What should I do?!

Added (1). Tommy thanks for responding! It was sold as a T-Mobile iPhone without a sim… I have tried to restore it from itunes and it hasn't worked.

You need to restore the device as new to complete the unlock process from AT&T. After that restore, you should see a message in iTunes saying Congratulations, your phone is unlocked. After that, you can continue with putting the foreign SIM into the device. Do you still have the AT&T SIM card?

The phone is not T-Mobile. It's locked to some other carrier. Get it unlocked first and then you can use the phone on T-Mobile. Check online for factory unlocking the iPhone, you can also mail me for help.

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