VVM Debt Collectors Need to Jack up the System

I am constantly being called in regard to a paid-up account with Telkom. I overpaid by R 2,400.00 rand, which I discovered in December 2018. I then lodged a case with Telkom and was refunded the overpayment. Telkom has sent me an account that shows a zero balance. However, VVM still calls daily up to 8 times in the morning and I am now just blocking their numbers. I have conversed with one of their consultants online, who asked me to send proof that the account was settled. I did this, twice, and to this day have not received any feedback from them. I have tried explaining this to their consultants but with no success. Either their call centre staff is not trained properly or there is an issue with their system. Be it as it may, surely debtor collects are supposed to check with their clients on the status of the account. I am hugely annoyed with the constant calling and the fact that they are not making an effort to sort it out on their side. Should this matter persist, I am going to appoint a lawyer to take them on.