I received a letter today from Moorcroft asking me to contact them. Not knowing who they were, I did so and they told me a debt I owe to O2 has been passed to them. I was unaware of any debt to O2. I previously had a dongle with them which I paid for monthly but I called them in August 2017, paid £7 which was the 'release fee' and haven't used it since. I remember the conversation clearly and have the email which gives the reference number of the payment. Moorcroft have said this is not evidence that I don't owe them money (the amount is essentially the monthly fee for the dongle since August-but I called to cancel the bloody thing!). They said O2 will not talk to me about it as they sold the debt to them but I know I don't owe it and that should be on O2's records, surely? O2 have made no moves to contact me at all as far as I am aware and my telephone number/email has not changed so there's no reason why they wouldn't use this if they wanted to speak to me. What can I do? I don't want to part with £100 for the sake of it but, equally, I don't want to find bailiffs at my door. I've just checked my credit file and sure enough it's on there. Any advice would be gratefully received.