There are a few incentives offered for customers new service subscriptions.  When working on installments tab, it displays a coupon option and Trade In.  You are able to find how many months of service can be paid with that value.

Example:  Phone Trade in Value = $200, plus get $100 in visa card for Directv Subscription.

 If your Directv pricing is $35, these credits pays 8 months of that service.

As a customer, you are able to find the AT&T's trade in value for your device.  Go to their trade in website: AT&T Trade In.