I bought a used iPhone 3GS and an unactivated AT&T sim card that I was planning on using for their pay as you go service. It won't let me activate it with the phone online and I'm not sure how to proceed. I called in and the rep said that the pay as you go service doesn't work with the iPhone. Do I need to buy a used sim card/goPhone and put that in the phone to make it work? Can I buy an already activated sim card from AT&T? I've seen some stuff online about getting the data plan to work, but right now I'm just worried about getting the basic functionality of the phone. Thanks!