Utterly disappointed

Very upset with vodacom pinecrest. They phoned me to go upgrade in august. I went in and i got my phone. They told me i would still pay the current phone until tge end of december. From january i would pay the new price. The last 2 months my contract has been R500 more than usual. After calling the call centre i find out that i have to pay an amotorised plan of R500 for 4 months to cover the coat of my old contract. I phone the sales man who sold the phone to me and querried this. He said no this is impossible and he will sort it out. After he looked at the invoice his tone changed and he said he told me about this and how he has never forgotten to tell customers about this payment. I argued with him because i had no knowledge of this and was not informed about this. After arguing for a while he has the audacity to tell me well there is no proof. Thats when i started to swear. Once calming down i asked who can i speak to regarding this matter. I was reffered to calling the accounts department. Awaiting to see if my matter gets resolved