Telkom staff cannot add

I lodged a query with Telkom regarding the incorrect billing on my account for May and the consultant calls me and confirms that she does not see anything wrong with the charges. How is it possible for my bill to go from R 299 to R 516.32 when there is a cap of R 10. I have my billing for April and it is clear that the system billing is wrong when I migrated my contract. The level of service of Telkom is pathetic as usual and the staff lack the skills (adding and working out pro rata costs) to assist customers. Telkom mobile will fail just like Heita did.