Accounts that is none existent or accounts they never closed when paid up??!!

Let's start with MTN, I have not had an account with them for 4 to 5 years, and now all of a sudden I have had calls from debt collectors telling me my account with MTN is in arrears. When asking them to send me proof of any account and where I signed any documentation they said they will look into that but the next week someone else calls with the same story, this is really ridiculous. they must get competent people to work in their account department. Secondly, I have been under debt review for the last 5 years and can not take out any contracts which is why I am asking them to give me proof that I have an outstanding account.