Incorrect billing on Vodacom account - 24 month contract

I have been a Vodacom client for years and years and never had any problems. This past 2 months, since my upgrade in March 2018 I had to phone Vodacom three times with regard to my Vodacom account amount on my Invoices (May and June). The first problem was a double deduction of my monthly handset instalment. Now that is sorted and I get my invoice for June and a amount of R7 217.54 (Equipment.....) making it a total amount of R8 779.61 to be paid at the end of June. This query I had to lodge twice. According to Vodacom it is a once off payment for my handset. Now my question: Who authorized that for a once off payment? I certainly did not! And if so why am I paying from end of March an amount of R328.07 for a monthly handset instalment? This is the worst ever experienced and this is not acceptable!