If you call ATT they will give you the unlock code so you ever gets this phone they can put in their sim and it will work when they put in the code.. You being the owner can call ATT will give you the code for free you just have to call. And then give the code to the winner.
As long as it has a sim card slot all u have to do is put ur sim card in and it automatically converts to your carrier. T-mobile has the 8900 so all you have to do is take it to tmobile and get it activated.