Blacklisted three months after cancellation?

I cancelled my Telkom account on the 30th of August 2017. I received a phone call to acknowldege the cancellation from end September 2017 with the reference number ********** 11a. Then I received an invoice in November to which I promptly rep**** that my account had been cancelled. A few days ago I tried to open an account only to be told I was blacklisted. So: 1. My personal reputation was brought to shame 2. I had to phone transunion to request who blacklisted me (a half hour phone call) 3. I REFUSE to phone Telkom and waste any more of my valuable time to fix an error which is theirs. I see this happens a lot with them. How on EARTH is this legal? How haven't they been sued for this? Also- I NEVER received a final demand letter (even though this was 3 months after cancellation)- no further communication. How disgusting. Telkom- please get your legal department to contact me ASAP and have this removed or I will be forced to take this up with MY legal department as you are hindering my business and personal growth.