Telkom billing for an account cancelled more than a year ago

I cancelled my ADSL and voice lines services with Telkom May 2018. I have since been called by collection companies regarding an outstanding amount, even though I should be in credit as per the date of cancellation.

The call centre agents cannot help me, logged various disputes, without any feedback and just get closed. Telkom processes and staff are clearly incapable of reading the original cancellation mails I shared with them.