VODACOM DEFAULT FOR A CONTRACT I PAID IN FULL AND CLOSED

I had a contract with Vodacom which I wanted to close and convert my number to prepaid. I was advise that I would need to finish off the contract, pay everything that is outstanding and then only would they convert the number to prepaid. All payment was done and I reached out to vodacom to proceed with converting the number to prepaid. This was done, the number is now on prepaid terms. However months later, I find out that vodacom listed the account which I thought was paid up, in default. I saw this in my credit report. Everytime I called Vodacom about it,I was passed between departments and nobody could explain why I have an outstanding balance yet I no longer have a contract with them and HOW did they convert the number to prepaid if the condition was to first pay off my account and cancel the contract???? My account was handed over to a debt collection company which did not bother calling me. I followed this up and eventually paid what was outstanding but I am still sitting with a bad credit rating and a default listing.