Overdue account

I have an account with Matrix. My first debit order was successful and the second one was not. Upon that I contacted them to change debit order date and it was done. After realizing that they did not debit my account I then called again and made the same changes that I made the first time. For 2 months my debit order was successful, I remember they debit my account twice at some point. I don't know what happened to the debit order instruction but my account no longer got debited. Today I receive a call from debts collectors that my account has been handed over. 1. No one at matrix bothered to contact me regarding my account, I have been begging for my account to be debited. 2. I am going to be charged some admin fee by the debt collectors that I don't even know of. 3. Can Matrix take back my account and follow the collection process. I refuse to be told that my account is no longer with matrix and that I must contact people that I do not have business with.