Samsung BlackJack Phone/Using T-Mobile

I recently received a Blackjack from a Friend who uses AT&T. Problem is, I have T-Mobile. I don't know what to do so that I can use that phone through my network. Does my phone need to be "unlocked" in order to use it with the T-Mobile network?? Any help will be greatly appreciated, as I don't even know where to begin.

Thanks,

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Call AT&T and get the PUK unlock code. I JUST did it myself. Tell them you're going overseas and you want to use your phone.