MTN Incorrect Billing

I had done an early upgrade in March 2015. A few days later my bill was already R650, I subsequently contacted the MTN store that the contract had been taken with. I went in to the store, the manager advised me not to worry about my billing as all the amounts will be taken off the bill. I have received my bill from MTN and it is R760.00. I have been mislead into taking a contract with an open line which the Consultant advised me that the my choice 100 is the same as the top up 100 which is clearly not. My record would reveal that I only take top up contracts. The store Manager is being very persuasive in terms of me keeping this unwanted contract because at the end of the day, it is more revenue for the store. I have placed a stop order on my account until this matter is resolved. MTN stores are not reachable at times as they do not pick up the phones. I managed to get through the Reservoir hills store at 2h15 this afternoon and the consultant cut the call on me, when i called back he said to me he was going to phone me back. He still does not understand that they have mislead me into taking this contract.The manager shouts from the background whilst on the phone.