Absolutely pathetic service Telkom

I have received absolutely pathetic service from Telkom and this is not the first time. I have taken out a month to month mobile data contract and cancelled it in July but it was only cancelled by Telkom in January this year. Now I have a default on the credit bureau because of this and Telkom cant seem to sought it out themselves. I require a settlement letter or a letter confirming payment arrangements but that also cannot even be done. This is affecting my job application as the hiring company wants written confirmation of this.