Sorry if this has been created, searched a while ago and did not find any related posts. I just got done with this entire process, I am using a Prepaid plan on my iPhone 4! Background info Got phone at 4.01. Used tiny umbrella to update my phone to 4.1 while keeping my baseband on 1.59.00 Jailbroke phone using greenpois0n, installed ultrasn0w and unlocked phone. I knew it was unlocked because i used a trimmed T-Mobile sim and that worked fine for 2 months. What I did I live in Chicago but go to school in Bloomington, IN, I wanted to order and test a sim to make sure I got service down there and that it would work. I ordered an unactivated go phone AT&T sim off of eBay for $3. Trimmed it and popped it into my iPhone 4 and got 5 bars and it read the sim card, because it said "AT&T". I got to Chicago, went to AT&T and asked to activate a prepaid Sim card, and told them I don't have the phone with me because it has not come in yet. They activated the sim card and handed it to me. I got home and trimmed it, and unfortunetly cut it too small. Everytime I put it in the iPhone it said no sim. However when I would put my T-Mobile sim in the phone it would give me one bar, indicating that it recognizes a sim card. (I am not sure as to whether or not the Sim from AT&T was not the right size or not working on the iPhone though) Luckily I still had the old AT&T sim card from Bloomington with me. So I called AT&T customer support and asked them to transfer my prepaid plan from the New one to my old unactivated sim, I told them it was damaged. I gave them the ICCID of the unactivated old sim, and it only took 2 minutes to transfer over. I popped it into my iPhone made a test call and it works perfectly. So for anyone who is having trouble, I would consider ordering an unactivated AT&T go phone from eBay, and try transferring your plan over, it only costs $3 for the sim I can now text/call on my iPhone 4 using AT&T prepaid.