You sure can! As long as you either purchased the phone at full cost or have fully paid it off if it was on an installment plan. Also, we put together a pretty awesome One stop shop for all the iPhone related stuffs. Check it out here.
Don't do it. It is a mess. I trade in my IPhone 6, There a receipt that they received the phone. But they say they didn't received it. So Unless you drive to the warehouse itself don't send it in.
Mine have been stuck in for the last 3 month and they billed the full amount each month.