Account Incorrectly Sent to Collections Agency

How do I escalate this complaint and receive confirmation that it is settled?

I cancelled my home security system and returned the equipment according to the instructions provided by Telus. I paid my final invoice. My account confirmed that I had a zero balance. The tracking information indicated the equipment was received at Telus. I have proof of receipt. Without contacting me, Telus sent a charge for the equipment to a collection agency who in turn contacted me last month. I disputed the charge.

I contacted Telus a couple times and once was passed on to Telus' collection department. The representative there said he was submitting an amendment to the collection and that the collection agency would receive it within a week or two. Customer service sent me an email confirming that I returned the equipment and did not owe money on my account. She gave me an unclear explanation about how this would be settled eventually and I did not have to do anything else. As I said, I have documentation proving I have returned the equipment.

The collections agency continues to contact me and have indicated they have yet to receive confirmation from Telus requesting that the account be closed. We are now three weeks into this.