Billed double and misled by sales representative!

I'm seriously ticked here. My phone was due for an upgrade in April and Vodacom called me at the end of February to do my upgrade. I said IMMEDIATELY that I am not going to upgrade yet because I do not want to pay double for the month of March and the sales person told me not to worry, I won't. The new contract will only kick in when the old one expires. Based on this I continued the call and went ahead with my upgrade.... at the end of the call the representative quickly went through the usual legal jargon.. Then I get my bill for March and I have been billed for BOTH contracts. I called the billing department to lodge a complaint. They came back to me a few days later and said yes they listened to the call and yes they agree that right at the beginning of the call the sales representative did say I will not be billed double but because I agreed to all the legal jargon at the end they are not going to do anything about it. OMG - at the very beginning of the call I said I was going to wait until April to upgrade and I would not have continued the call if the sales rep did not assure me it would be fine I would not be billed for both contracts for March. How is this OK?