Contract Cancellation Dispute

I am actually writing this out of frustration, just got a call from Telkom debt collectors, after my contract that ended in August, they telling me I owe 439, mind you I called and wanted to cancel the contract and was advised to go to the shop, I did go to the shop not once but twice and they confirmed that my contract was cancelled and I was on prepaid.

Imagine paying 439 for a service you didn't even use, again that the agent from debt collectors is telling me the contract wasn't cancelled and its on a month to month basis. FOR GOODNESS SAKE I cancelled the contract at the shop.

Telkom is full BULL****