Homeloan problems

My Homeloan/bond is paid in full, in fact with over R2000 overpaid. For a few months now FNB is taking the monthly service fee off my cheque account instead of reducing the overpayment. I have phoned, emailed and they just do not respond. The phone was answered once by a human that was supposed to transfer the call instead cut me off. After that every call, I do the verifications, wait for ages only to get cut off again. I have sent 3 emails one on 27/08/2019; another on 04/09/2019 and today 16/09/2019 I have received confirmation they were received.