Sent to credit company after payin bill in full

I took out a contract with Telkom mobile. After a few months I changed my banking details and forwarded them to Telkom to update. Telkom failed to update them and hence did not debit my account for 3 months and I had to deposit the money physically into their bank account. After having paid the account in full, they sent my details to a Credit company to have me blacklisted. I called the credit company and they said I would have to send them proof of payment. I let them know that I was unable to visit the bank to get proof due to my work schedule. I then called Telkom to have them rectify the issue as my account was fully paid and I do not understand why I was sent to a credit company to become blacklisted. Telkom call center agent Zanele said she would log a case and gave me case number ***1. The case was opened on 14 April 2016 but I am still receiving messages saying that I have been blacklisted. My issue is they are proving to be very unhelpful in rectifying the issue and I was sent to this company even after I had confirmed with Telkom that my outstanding balance was paid up in full.