http://appadvice.com/appnn/2014/11/att-is-now-offering-a-50-billing-credit-on-iphone-purchases-made-through-apple

The rules of the promotion are as follows:

• The iPhone must be purchased on an existing line as an upgrade.
• The iPhone must be financed on an AT&T Next installment plan.
• The iPhone must remain active and in good standing on Next for 45 days.