CONTRACT NOT CANCELLED AND BILLED INCORRECTLY

I cancelled a contract on 22/3/2016 by paying the full settlement amount as per premature cancellation quotation. The contract was to be cancelled on 31/3/2016. However, on 1 April 2016 (Friday) the contract was not cancelled and I contacted Vodacom cancellations dept, they just apologised and requested that we not use the allocated talk time, data and SMS's until 4/4/2016 as the contract was not cancelled on time. On 4 April (Monday) the allocated talk time data and SMS's was removed from the phone and it was switched to a prepaid package. However, on 11/4/2016 when I received my statement, I was billed again for the talk time, data and SMS's which was loaded incorrectly onto the phone payable at the end of April 2016. After numerous phone calls on 4/4/2016, still no joy and the cancellations dept is now unable to assist in correcting this as it is now a prepaid package. I logged a query with the accounts dept and still nothing has been done. I was promised a SMS confirming a dispute that would have been logged, till today, 19/4/2016 nothing is received