Will iPhone 6 get BLOCKED after non payment of Bill? Conflicting info online?

A work mate has offered to sell me her iPhone 6. She's been with o2 for 12 years and can no longer afford the bill so she's stopped paying. Companies lock the sim and not the phone however people mention blocking contract phones when people sign up to a contract, order s new phone then claim on insurance. That's when the block happens. A company won't block a phones IMEI number and then chase you for the debt as you're being chased for a debt for something that won't work. It is my belief that a phone is blocked when filed as lost or stolen however non payment of Bill it's the sim not the handset. It's been unlocked so I'll just stick my own sim in etc but reading up about blocked phones is a worry incase I buy a brick but by the same token if it's just scaremongering I won't be buying a brick? Can someone please clear this up for me? I would like the phone and be able to help her out by buying it but… Is there really a risk of a block?

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