unjust account hand over, credit score affected

Telkom has done it again! always so disappointing with their service. I cancelled my telkom landline on 5 sept 2019, i got a confirmation number. Telkom did not cancel it & I got a bill. When i inquired about this, I got an email apology & Telkom credited my account. Now i have been handed over to debt collectors & my credit record shows non-payment for months that followed. I would like this rectified, i have tried to get hold of telkom to no avail. they are unprofessional and i will never use them again. i will tell everyone i know to never take their deals.