unreasonable account

Good day, after being harassed by Telkom mobile's consultants, and being promised that I will only be paying for data and airtime after my contract have expired, I decided to keep the package as it seemed very reasonable, after doing that I have received a very unreasonable account now, with no luck or help from Telkom mobile, I am now fed up with them, the amount that they billed me now is even more than what I paid when I was still paying for the device and data and airtime, so how did they manage to calculate that? in October 2015 I received an account for R235.76 which I'm more than happy to pay, but in November I received an invoice with the balance due as R 974.77, I'm getting R150 airtime and 2.5GB data, so how did they get to this amount, I would like feedback and they can cancel this contract with immediate effect, I agreed to a month to month contract but this was not what was explained to me, Mildred from Telkom told me that I will only be paying for my data and airtime, but now I'm paying for whom knows what else as the device is already paid off and not even working anymore!!!!