Atrocious Service and record keeping

I have settled my account in full and received a paid up letter confirming this. I have now started receiving calls from the call centre advising that I have an outstanding balance. When I settled my accpount with them in June 2018 I sent proof of payment and received my paid up letter on 05 July 2018. Suprisingly they have on record that I have an outstanding balance of over R1000.00. Many calls to the call centre, some calls were dropped in my ear when asking to speak to a supervisor, and I finally received a name of the lady that was going to call me yesterday 26.11.2018. Berverly requested that I send proof of payment and the paid up letter again, which they should've had on file by the way. Shocking to say the least. I am only hoping that the matter will now receive the attention it needs as this is now bordering on harassment. How do we resolve this Easton Berry complaints team?