Still being billed after cancelation

On 20 August 2015 my contract with telkom ended and I did phone them to cancel it and had to pay the outstanding amount of r1079.50 in order to cancel it. On my bank statements it shows that the transaction was successful, so I didnt bothered to follow up on that. Now I got a statement from telkom that shows I have to pay r1079.50 plus september and now october all together. I refuse to pay septembers money and the cancellation fee of 1079.50.I need someone to sort this BS out! I will NOT pay that amount of money! Thats why I changed my isp to openweb and did not even use telkom after august.