Are you sure flaperon ? If you get a £500 phone on a £50 pm contract and never pay how is the phone yours ?
The phone is yours from the moment you sign the contract. It’s basically a gift. If it still belonged to the operator, then any fault would be the phone company’s problem to fix. If they couldn’t fix it quickly, you’d be able to leave the contract early.
It’s usually buried in the terms and conditions somewhere but they make clear that the phone is yours. I’ve used unreasonable price-hikes in the past to cancel 24 month contracts six months in and kept the phone under this argument.
* caveat: I have no idea how this works with the O2 thing where you pay separately for the phone.