Wrongfully blacklisted by Telkom

My father cancelled our Telkom line in 2018. Telkom continued to bill him. We made numerous calls to the call centre and everytime they said, the account has been closed. Eventually, my father paid the account (12/04/2019) after getting a letter that they were going to hand him over. Telkom in the meantime, reversed the invoices and the account was in debit (meaning Telkom owed my father). But when my father went to the bank, he found out that he has been blacklisted. We went to Telkom and ask them to sort it out. We phoned the call centre but no one seems to be able to help my father, recover his credit status. Seems as though Telkom handed the account over to debt collectors, Blake Collections. Blake Collections is also of no help, saying the need instructions from Telkom. The lawyer even adviced that it's no use handing Telkom over. They are useless. Now what are we to do and how do we recover my father's name?