In some cases, you might find that rather than selling or disposing of your old iPhone, you would like to reactivate it, either for yourself or others. To reactivate the iPhone, you need to acquire a new SIM card for the device's cellular network. Once inserted, reactivating the phone is easy with iTunes' automated reactivation protocol.
Take the old iPhone into an authorized AT&T dealer and tell the representative you would like to reactivate the phone. She supplies you with a SIM card that transfers your plan over for use on the old iPhone.
Insert the new SIM card into the SIM card's slot, located on the top edge of the iPhone near the headphone jack and "On/Off" button.
Plug the iPhone into your computer using its USB adapter cable and launch iTunes.
Allow the device to link with iTunes --- this could take a moment. When the device is linked to your iTunes profile, the program notifies you that the device is not recognized and that it needs to be reformatted. Click "Continue" to allow the device to be reformatted for use with your computer. When this process is complete, your iPhone is reactivated and ready to use.