Final Demand on paid up account!

After 18 years of good service, I have received now emails advising my account is overdue - I have sent payment advices showing it is paid in full. I have now received a Final Demand by email requiring a month's payment and a cancellation fee of +/- R4****! When checking my account online it shows my 'SIM is permanently disconnected' and I can no longer receive or send phone calls or sms - the 'service is suspended'. I have sent at least 4 or 5 emails requesting this accounting error be resolved and my service and SIM be reinstated. I have received automated acknowledgements, but no response or explanation. It is possible there has been ********** activity, so I have reported it as such, too. I now wish to know urently what steps I can now take to have my service restored.