Can I just use any phone for AT&T?

I want to get a new cell phone but I don't wanna pay a ridiculous amount for one. So I was gonna buy a T-Mobile cell phone from my friend and change SIM cards. Is it that easy or do I have to "unlock" it?
 
you have to unlock it.. but becareful when unlocking because if you get the code wrong too many times the phone will have to be taken in to the store..
 
you have to get the phone unlocked

or just buy a cingular phone off ebay
i got a brand new blackberry curve, priced in store @ $300. i got mines for $150.
 
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