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.