Cancelled account incorrectly handed over

I had a data sim contract with Telkom, I cancelled it this year in April, I was provided with the Cancellation letter and my account was not owing. To my surprise in October I received calls from debt collectors acting on behalf of Telkom, I am shocked because when I closed the account it was out of contract and I was not advised of any arrears. I will be taking this to the telecommunications ombud.