It wont probably pay it off in full, you may get up to £500 back for the device depending on condition.
But you have to pay the upgrade fee upfront.
Current Phone: Sony XPeria Pro-I and Motorola Razr 5G I also have a Planet Computers Gemini
I work in IT as an Enterprise Messaging and Collaboration Tools Administrator (Office 365, Slack and much much more) and manage hardware both Mobile and Desktop / laptop, my desk is a bit like PC World
-- We are all customers and dont have access to any o2 accounts