CANCELLED MY POSTPREPAID 6 MONTH WITH NO ACTION TAKEN, NOW MY ACCOUNT HAS BEEN HANDED OVER

I have been using MTN Post prepayment option for Data, but the network was terrible, and I couldn't even use it to connect on my laptop since the network was always poor. it has been more than 6 months i cancelled and return the router at Jubilee mall branch at Hammanskraal since well it was a month-to-month payment option. but no action was taken on my account, now my account has been handed to credit bureau, currently in arrears which is affecting my credit score badly. when i called the customer care today i was notified that my account is active still, whereas in my knowledge the account should have been closed long time again.