No response to cancel contract

I phoned Telkom 081180 to cancel my contract, it says I must send a written cancellation to ***. I also tried to use the Telkom app to try and cancel. They let me know twice that a fault has been cancelled and the matter is closed. When I try again, unexpected error, try again later. After I wrote a letter of cancellation, I received a notification instructing me to write to the address above which I done already. I will now stop paying the account and see where it takes me.