Incorrect monthly fee

I upgraded in December on Vodacom's website and they showed me and sent me an email confirming that my total bill including insurance will be R1499. Since I saw my first bill during December I started phoning and emailing them as they are charging me R1850 now. After numerous calls and emails they are still deducting R1850 per month from my account and the Vodacom employees that I'm emailing are ignoring me. That means they already owe me R700 and it just keeps on adding up and they keep on ignoring me.