Ok so here's the deal. I'm getting a 3G off craigslist in a couple days. A woman is selling it who's had it for less than a year. Anyway, I'm getting a new AT&T sim card in the mail and I don't have service with AT&T. Once I get the phone and sim card what do I have to do after I call to activate the sim. Do I just pop it in the phone and set up a plan or what? I would think i don't need to sign any contract with AT&T right, since i already have a phone. Thanks.