Irreconcilable billing and terrible customer service

After my overseas trip in July, Vodacom billed me over R8000 on data for roaming. This i disputed as i was mostly on wifi. A guy by the name of Marvin Timm responded via email saying my bill had been reduced to R5216. I still disputed this asking how come my set limit on my account did not "kick in"...i received no response from Vodacom after numerous attempts. At the end of Aug i was billed R19k for the same trip in July (this i reversed from my account). I disputed this and it was reduced to R7k in Sept (which i also reversed and only paid R2.5k). No one has been able to answer my questions on how they get to the different billing amounts and why my set limit did not apply. All i get is threats of legal action from their staff that have zero customer intellect.