Bad Service

On the 2nd of October 2018, I logged a request on the Telkom website to cancel a contract that was expiring on the end of October 2018. 3 months later I'm still being billed for this number! when I call I'm told "no sir you need to pay the outstanding amount for us to be able to cancel this contract" Why must I pay for the 3 months that I have not been using the number for them to cancel the number whereas it is their fault they didn’t can cancel the number in the first place.