Most cell phone companies will be willing to compromise with you. If you buy the phone and then take it into a store, they will probably make it work.
I'm not garunteeing anything, but I have Verizon and I know other people with the same phone I have (which a Verizon exclusive), but have different cellular carriers.
You can use the Blackjack 2 on T-mobile. Like others said you need it unlocked. Unlocked means the phone can be used on any GSM network. You get the phone unlocked by either calling AT&T customer care and asking for the unlock code. If you meet the requirements they'll give you the unlock code. If they do not give you the unlock code, you'll have to go to a 3rd party company to unlock the phone. Sometimes it is just best to buy the Blackjack unlocked already to skip all the hassle of trying to get it unlocked