Credit derogatory mark

I noticed a derogatory mark in my credit score from Telus so I recently contacted them to ask about details and history and they told me they were unable to provide the bill and details as my account has been locked and turned over to creditors. Granted, I haven’t had a home account with Telus in 3 years and my last bill record I have shows a clear balance and closed our account as we cancelled services. So I called the creditor they provided me details with and they also said they have no bill details to provide me and that I need to contact Telus. So I am being thrown in circles with no answers and now this is affecting my credit. How do I get this removed from my credit if no one is willing to work with me?