Account Handed Over

I had a cell phone contract with MTN that I terminated in March 2015. My contract number was then migrated to prepaid. I then ported my MTN number to Vodacom network. A few months later, I noticed on my credit report that MTN has hand over my account for an amount of R810 that i apparently owed MTN. I called MTN to inquire about the issue where i got the confirmation that my account has indeed been handed over to the legal department. I request to be taken through to the legal department and I was told that they do not calls but one of their representatives would call me back of which it has not happened to date. I then paid the R810 that was said to be outstanding into MTN account. My complaint is that MTN never notified me in any form that there is an amount due to them. None of your legal department has given me a call regarding my inquiry. After the payment that I have done it still reflecting that I owe MTN on my credit report and that is negatively affecting my credit score very badly. I do not understand why would MTN approve porting my number to Vodacom if i had amount due to them? Why would they convert my contract to prepaid if there was an amount outstanding?