Poor service

In June 2019 I was contacted by Mondo to take out a data only sim contract through MTN. Last month I noticed for the first time that I was billed R299 instead of R199 as telephonically discussed with the agent. I called and they said they had to investigate by listening to the recording, they called back after a week and said I was correct about paying R199 and the billing issue was with MTN and they will sort it out. I called them again for feedback and the person I spoke to said the matter was closed and was in favour of the agent who sold me the package, yet I am still being billed R299, they were suppose to call back with a solution but nothing so far. ***