Cell C overcharging against our agreed amount. Which was highly specific and understood. A false agreement

Cell C overcharging at will. I went into Cell C Rosebank and spoke to Charlene. After an extended discussion of choices, we agreed on a contract and she explained to me that although I am changing to R1000 a month that will cover all voice, sms and 10GB data i will need to pay R1770 for the first 8 months to complete my contract with phone. I will then go to R1000 in May 2020. Yet this month my bill was R2440 (about) and now she says next month will be the same. and following it will be R1800. This is not what we agreed. This is a false agreement that Cell C needs to rectify to what was agreed upon by their representative.