Billing Issues

I app**** for a 20Mbps uncapped ADSL from Webafrica. It was installed, but what I did not know and was not notified off was that Telkom could only provide a 4Mbps in that area. Meanwhile I am being charged for a 20Mbps line. I've been on the phone with their technicians a few times about my line and no one tells me this, until I finally got to a tech that could tell me why I am not getting the speed I am paying for. At this point I already paid for two months of 20Mbps. I phoned the billing department and explained the situation, and they said that they would rectify this. I received a credit note from them and thought that all is sorted, until suddenly a ridiculous extra amount gets debited off my account. No explanation, no invoice. Then this morning I receive an invoice for December, and I am now being charged for a 4Mbps AND a 20Mbps line. And this is after how many phone calls and they cant even get it right.