Closed account- final invoice never sent, debt forwarded to collections without notice

We moved nearly two years ago and transferred our home security service to the new owners of our old house. The final invoice was never issued even though we remained customers for our other services (like internet and mobile). We also never got a notice it would be sent to collections. Our first idea this had even happened was a call from a collections agency. 

The first agent we reached said they ‘couldn’t email or mail’ the invoice because the account had been closed (that’s not actually how accounting, emails or mail work). The next one acknowledged it was their fault and we should pay the agency and call Telus back and they would take care of the rest. 

This has been weeks. It has of course damaged our credit score because Telus needs to be the one to do something about them sending something in our name to collections. 

We have served them notice we will be escalating this to the CCTS. Heard they would contact us within 48 hours. A week ago. 

They are now telling us it’s our problem to take up with the credit bureau. 

Which agency did everyone else have the best luck getting results from?