Cant locate my payment

Received a bill in July 2015 made arrangements to pay R1000 end of July and settle the account end of August 2015. Made the R1000 paymeny on the 30th of July and up till totay they keep sending me bills for the total ammount. On numerous times i have phoned them i have sent proof of payment. They will not phone me but will only send me an account via mail. I have been following up weekly and the only response is that its with the banking department and that is the same response for the past month. I would like to settle this account but cant untill my payment has been found.