Cellphone contract

I had a contract with vodacom where I was paying R395, I received a call from vodacom where proposed to reduce my contract to R345. They stated that they are converting my sms to minutes. Only to find that they have actually increased it to R670. I contacted them to enquire. They are telling me something different from what we agreed upon over the phone. I asked them to reduce the installment back to the initial one but they are failing to do so. My account is in arrears as we speak as I cannot afford to pay such installment.