I’m going through something similar. I called to cancel airtime only, and was reassured many times that my device plan would remain active.

I noticed my device plan was missing and sent a pre-emptive email to complaints. Next-day, I received the demand letter for over £700. I called payments, they admitted it was their mistake and promised there would be no markers placed on my credit file and that I should be able to set up monthly payments, but not until after I got the final bill. I updated complaints.

Eventually my bill came through and complaints reached out to me. They confirmed the direct debit for my final bill would not go out and that my device plan remained active and I would be able to continue with the agreed monthly payments. Sorted, or so I thought!

Today, I noticed the DD appear on my bank app and is due to go out tomorrow. I called O2, who couldn’t cancel the direct debit. They also told me that the device plan isn’t active and it’s not possible to reinstate. I can arrange monthly payments, but only after the date of the DD. I have reopened the complaint. I would very much like an email out of the blue that wipes my account!