First, make sure you do a new backup of your old phone. When you plug in your new iPhone 4, it will ask you to register. Do this first - it activates your warranty. Next, you need to restore the backup for your other phone. This will not delete anything - it simply sets up your new phone so it will be just like your old one. After you are all set up on iTunes, go to www.att.com/activations and follow the instructions to activate your phone. You can also call 1-866-895-1099. They will ask you to turn off the new phone for about 5 minutes. Turn it back on and see if you have service. If you don't have it right away, just give it time.