Ok...so...yea? needing some help?

a couple weeks ago i was told by at&t that i couldnt get a certain phone (the blackjack) because i wasnt in that plan. today, i came across a blackjack, the exact one i wanted, and the person i know is trying to seel it. wat im wanting to know, is that if i buy this phone from my friend, will the phone work with all my stuff and my old sim card? will i lose anything? will at&t allow that to happen? gimme some details if u have ever had this happen to u, and thanks guys