Outstanding balance for an account that was ended and paid in full

I received another email from a collections officer regarding an outstanding account balance for an account I closed a year ago that had no outstanding balance when it ended. The last bill I was given from Telus was for $0. I also checked for any mailed bills that I could have received and there are none. There seems to be an Issue on Telus' part, because I shouldn't have been sent to a collections officer for a Telus account debt I never had. I need this to be rectified as soon as possible, I do not want my credit score affected by Telus' mistake. 

Thank you, 

I hope you can help.