Impossible to cancel a mobile contract if you are behind on payment

I have contacted Telkom Mobile to cancel my contract and they have informed me that I am unable to cancel my Telkom contract due to the arrears of my account my contract has expired more than 6 months earlier but I am still unable to cancel the account I was informed that I have to pay everything to open the account for them to be able to cancel this account I do not have the amount outstanding on this account as I was under the impression this account was canceled after I phoned them