I am looking into buying a used iPhone 3G off eBay. I right now have an LG Xenon for a cell phone. I do NOT want a data plan with the new iPhone. I already have AT&T and my LG phone uses a 3G SIM card. I am NOT going to jailbreak it. Can I just put the SIM card I have already in the iPhone and the iPhone will work right off the back? Or will I have to get the phone regesterd to my AT&T account? I have data blocked on my SIM card so that I don't go on the Internet without meaning to.