Huge increase in monthly billing

I have been with MTN for almost 2 years, my monthly billing has always been around R320 a month. I was shocked when I received a bill for R1240 (June) and R1350 (July), when I queried these charges they said it was data usage, which is not true as I use WIFI 90% of the time and have never changed the way I use my phone from the previous months. I told them that they should review my billing history and will find that it is always around R320 a month. Since MTN had their strike and return to work billing has been incorrect, a few people have the same complaint. I have even spoken to MTN staff at the Garden Route Mall that have had a lot of complaints about the same issue, but are unable to give me a solution as MTN customer care can't resolve the problem. So tell me MTN what now? In good faith I paid R320 for June and R320 for July, I am now going to switch to prepaid and still continue to honor my contract for R299 a month until it expires or until MTN resolves this issue.