Account billing

I requested a settlement for my cellphone contract which I was provided and also advised of termination fee . I was then transferred to cancellations department same day to cancel. Payment was done the following day and proof of payment was sent with call reference number. Contract was cancelled , however a month later Telkom decides to go and debit my account for a large amount advising me its the last invoice with termination of contract, how! I called in today and the agent did not make it easy to explain the reason why I am billed after contract is settled. I am not will to pay amount because when I requested settlement all the amount that was due to settle the contract was supposed to be transparent however am being told when I have cancelled the contract.