No response from anyone!!!

Since July I have been trying to cancel a line as the business was closed down. After numerous frustrating calls to 10217, no one answers and puts the phone down in my ear after holding on for 20 minutes. 3 letters has been sent with no reply. Get an invoice every month for a phone that has not been used and the amount is R2400??? Please tell me how this is possible? We will not pay the account!