Incorrect Data Billing

I was billed for R1699 for data usage on my cell phone contract no *** in January 2015. I logged a query, reference A6-W8SS-7M2IN, to dispute the charge but I am not getting any satisfaction from Vodacom. For the month of January, I had over 128mb carried over to February, so I do not understand why Vodacom claim I used R1699 worth of data. Secondly, I have a R50 limit whereby my line is soft-locked if I exceed this limit. How did Vodacom allow my account to reach R1699? I have another line, *** which exceeded the limit but made two purchases of 500mb at R39 each, after I spent R42.28, which is within the R50 limit. According to my contract and extra data usage, my bill should have been R835.38 but made a payment of R1050 as suggested by the Vodacom Consultant. I am not happy as I am receiving threatening messages from Vodacom and my line is occasionally blocked. One consultant presumed that this data usage could have been in December 2014, but why billed in January and why line *** shows January usage. I would like Vodacom to reverse R1699 billed on my account as this was incorrect and was an error on their side, I am not taking responsibility for the charge.