What? If you paid off, in total the phone, it is yours period. That's paying all 20 payments.
Sounds like you have not reached the amount of payments to upgrade. When did you start Next program with this phone and which payment number. There were choices.
If you are on the 12/20 next plan and paid 12 payments you can upgrade and return the current phone skip paying the remaining 8 payments. That's $379.60 balance for my 6 Plus 128 gb. If you have not paid the 12th payment you can't trade in. When you order the phone they give you the option to pay up to the 12th payment plus tax on new phone in the upgrade process.
Let's get the terminology strait here. Paying off the phone means, paying the full balance of what you owe. If it's a 128 gb 6 Plus then $949 plus the tax you paid up front. That's when you OWN it.
Paying to the point when you can trade in for the new phone means you return current phone after making the minimum payments, receive new phone and start the process all over again.
I read AT&T has three varying length Next alternatives and one choice where you can put more down up front to reduce the monthly payment. In all cases you pay no interest. Completely your choice from start to finish. I am going to pay my twelfth payment and turn phone in. I've had eBay problems in past won't use any more. Sold another phone direct to have person come back complaining because of a tiny cosmetic scratch on case. Pain in ass for a few extra bucks, that's me. You may do as you wish. Point is With Next it is completely above board, no hidden costs.