Telkom does not care for the consumer. They overbill

I signed a R99 contract with telkom in December 2017. 1. In June I asked why my amount doubled and was told my account had missed the May amount so I EFTed R300 (which equals 3 x payments). 2. In July they still debited R207.08 even though I had paid in advance. 3. I still got billed more than the amount needed. I asked why the amount kept fluctuating (R128, sometime R100.52, sometimes R99) and was told there was a subscription linked to the account but they cleared it (fictional) 4. In October I got billed TWICE. 5. In December it was over R400. 6. This morning they tried to take over R300 and I just had to reverse it 7. My argument is that I have paid in advance- enough to cover me til April which is why I tried to stop the debit order after the December run 8. Please explain why I am being billed incorrectly