Poor account management

In 2017 i took out one of those fancy two numbers for one contract specials, first mistake. Last year i canceled this contract as i no longer required the numbers to be active and planned to go to prepaid. However only 1 number was canceled by my dear friends at vodacom and the other was kept open. Needless to say they continued to charge me on the second number, i even had a discussion with a consultant afterwards informing him of the error with a promise made to me that this would be rectified. Fast forward nearly 8 months later I receive notice that this account has now been handed over and is in arrears of a massive amount. Not impressed as this affected my credit score and i was not even at fault.