Vodacom Incorrect Billing! overcharged for 5 months and counting

I attempted to conduct an online upgrade for one of my vodacom contract, however the site freezing, hence I utilised that chat option and was assisted by one of the vodacom agents, who processed the upgrade for me. The contract that I upgraded to was for an Iphone 6 at R 259 per month. it is now 5 months later and during these 5 months,I've been charged R428! The agent assisting me also advised that the R259 was inclusive of VAT (I have a screenshot of this conversation). I have called numerous times without this being resolved. I really cannot wait for these contracts to expire so that I can move networks. Vodacom. Pathetic service.