Billed more than six times my SPEND LIMIT!

I have had the absolute worst service from Telkom. Let's take it back a few months: I opened a mobile contract with Telkom, they lost my ID Card when making a copy of it and I HAVE YET TO HAVE ANY RESOLUTION ON THAT MATTER! But that is not what this is about anyway... The agent set my spend limit to R50 - this appears on my contract. My January invoice bills me for R315... how is that even possible if the limit is set to R50? I call customer care and ask them that question. Of course, they don't know. I ask them to check the system and they tell me that my spend limit was actually set to R500. Surely TELKOM needs to take responsibility for their error, as I will not pay more than the R50 which was set, and in writing on their company documents.