Incorrect contract resulting in incorrect billing

During March I upgraded on one of the phones on my contract. I requested Johannes at Stoneridge for the cheapest possible top up. Johannes had to do a sim swap as the sim card was lost. He did the sim swap on my number instead of the upgrade and I had to go back a week later for him to do the sim swap on the correct number. I used the new phone/sim during that week. My account for March and April was fine. When the debit order for June went through I realized something was wrong and on checking saw that the bill for May was R1,789 and the account for June was R3,821. I immediately removed the sim card and went to Stoneridge to query this. The consultant advised me that the contract is not a top up and she immediately logged a call limit the usage (ref ***) and also logged a call in respect of the billing problem (ref ***). No one bothered to call me back and I was now informed by the call centre that the store must sort this out. I was also informed that MTN does not take responsibility for the mistakes that their staff makes and that they cannot reverse the charges. I need to proof that I asked for a top up when I did the upgrade.