R5000 overbilling query still not answered

I have a consistent record of no overuse on my monthly usage and contract of R121/pm. Suddenly slapped with a R4000+ bill in a normal month. Since then i've written numerous mails with absolutely NO response. I've made ad-hoc payment monthly fee after i reversed debit order. This month, they just take it off again. Still no action whatsoever. The same happened the following month. And the same happened to family members. also no answer. I've shown my attitude, they clearly do not care but love to take the money!?