New offer of R1700 for data isn't what it seems

Vodacom only doesn't do business online, only by mobile calls. I was called and offered 100 gig a month for R1700. I asked if that amount was in addition to what I was paying and told it wasn't. I understood the amount paid would be instead of what I was already paying and agreed. Got notified next day of my new updated account and it was in addition to what I am paying not instead of. I have by law a cooling off period so I immediately called to cancel and they said they would but when I checked back the account had been cancelled but only from next month so the end of this month I will have an extra R1 700 to pay which I can;t afford. I called and protested again and the consultant said she would escalate and call back. She hasn't yet.