How to Get a Used AT&T iPhone Activated
To activate a used iPhone on AT&T, you will need to visit an AT&T store and obtain a new SIM card. Have the technician at the store put the new SIM card in your phone, then take the phone home and activate it through iTunes. Make sure that the information and data from the person who owned the phone previously has been erased; if they have not already done so, you can do it yourself in the Settings app, under "Reset."
Download or update to the latest version of iTunes if you don't already have it.
Connect your iPhone to your computer using the USB docking cable. iTunes automatically detects your new phone and displays a "Let's Get Started" greeting. Click "Continue."
Click the circle next to the option that best fits you. If you are already an AT&T customer, choose to either add a phone to your existing account or activate the phone on an existing line. If you are a new customer, choose to activate either one new iPhone or multiple phones on a family plan.
Enter the required information, which will vary depending on whether you are a new customer or an existing customer.
Choose a service plan.
Sign into your Apple account, or create a new one if you don't already have one.
Agree to the iPhone terms of service, then agree to the AT&T terms of service. Confirm that all of your information is correct and click "Continue."
Wait for a few minutes while AT&T activates your iPhone. When it is ready, the iPhone lock screen will appear, and iTunes will display a screen for setting up the phone and syncing your media.