I'm considering buying an iPhone 3GS off of ebay to replace my old phone which is on at&t, the only plan on my current phone is a $30 voice plan, I would like to keep it that way. What exactly do I need to do with this iPhone to keep my plan? (I also plan on using my current sim card) Do I need to jailbreak and/or unlock it? I was also wondering if I could use a jailbroken iPhone on a pay as you go network, specifically at&t pay as you go. If so, how could I do this? Thanks for your all's help!