Telkom Continues Billing After Account Cancelled

In January, I cancelled my Telkom service (ADSL Line and phone line), and paid the pro-rata amount to settle the account in full.<br> <br> Today, I get a bill from Telkom regarding services rendered on March 2016 for R399. I have not even been a Telkom client since Feb!! Why does Telkom always, always fail to process things like these correctly? I refuse to pay this bill, which will likely result in Telkom putting a mark on my name because they can't get their house in order.