Verizon would accept your phone with their trade-in program and give you a gift card in exchange that you could use to pay towards your bill, but they don't usually offer market value for phones. You can appraise it on their website or at a store to find out the amount they'll offer. Selling the device to a 3rd party, or site like Gazelle.com is usually a better deal, and you could use that money to pay towards your bill. Or if you go to another carrier you could keep your phone and use it on a competitors network since it is unlocked. Also some carriers run specials sometime to offer compensation for switching over to cover your expenses for switching, so that could be a good option too. You mentioned 'paying off your phone' so if you are on a monthly device payment plan you're correct, those do not require a service contract.