Blacklisted without a contract or agreement

I was blacklisted by Telkom when I had no agreement or contract with them. My rights as a consumer were violated. The problem stems from a canceled contract in July 2018. There was no network coverage and Telkom did not hassle, the contract was canceled and the cellphones handed back. I received a cancelation code, and that should have been the end of the matter. Problem was that I was being billed month after month. I provided my cancelation code, month after month but nothing was done. Eventually, I received calls from a collection agency and Telkom decided to blacklist me. After that, I was unable to conclude other contracts. The matter was resolved after it was claimed that the account was paid, which was not by me. Should Telkom not have followed some legal procedure before taking such drastic action. Should they not have made sure of their facts before doing so? What sort of people work for them?