If you return the phone, they will terminate your contract at the same time, so you will lose the number associated with it.
o2 dont do partial returns.
Your option is to pay off the device plan and the move to a sim only contract and then use the new phone with the same number.
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