cancelling my contract

We had a phone conversation about a account that need to be cancelled. On the acount letter it states a fax no that need to be used to fax prove of payment. So we did on 25/09/2019. We also faxed the cansellation letter to same fax no. Now after several months they said the cansellation letter is faxed to wrong fax no. Nobody indicated it to us. Now we must pay again R600 befor the cancel it.