Start off by making a complaint.
Use Resolver
https://www.o2.co.uk/how-to-complainIf on refresh, you can buy your device and leave for free.
https://community.o2.co.uk/t5/Pay-Monthly-Pay-and-Go/How-does-O2-Refresh-work/ba-p/1014093If not on refresh, you will need to get complaints to allow termination of contract to avoid fees, but they may ask for your phone to be surrendered first.
You can also try the new app called O2 Ask as it allows calls over wifi.
I have no affiliation whatsoever with O2 or any subsidiary companies. Comments posted are entirely of my own opinion. This is not Customer Service so we are unable to help with account specific issues.
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