CELL C: BLACK LISTED ALTOUGH CONTRACT HAS BEEN CANCELLED

I terminated my CellC contract by telephone in January 2021. The lady told me that I did have to go through Feb 21's stop order, which I did. I also received an SMS from CellC confirming that my contract had been canceled. My CellC number was by then also "pay as you go" and consequently I was able to enter into an Mtn contract with the same number. After that, I had to reverse a debut order several times, which went through without my approval. I was also contacted several times by a collection company, who I repeatedly told/informed that my contract had been canceled. I even provided the above mentioned SMS as prove that my CellC contract had been canceled, though with no avail.

In April 22, I app**** for financing for a car and my application for financing was rejected because the CellC account shows as a "default account" on my credit score, in other words; CellC blacklisted me although my contract has been canceled and I don't owe nothing (stop order for Feb 21 - last and final payment--went through on my account.

Would you please advise me on how and what I should do to get my credit record clean again?