Wrong billing

I was called to say i can upgrade my cellphine contract which i said its fine i upgraded and was told to pay R308 a month last month was the first month to pay which i was told there was a admin fee so we paid it i now get my phone suspended i phoned them they said because of early upgrading i have to pay R467 per minth until november because of early upgrading. I was not told this when they upgraded my phone. I asked the agent if its R308 per month and he said inly R308 per month when i was upgrading. The previous time i upgraded i also had wrong billings i am not happy about this because i have a budget every month and this is not on. Why tell the client its one ammount to find out it is not. So either cancel my upgrade or make me pay what i agreed to pay per minth.