Dates and cutoffs changed, no prior notice!

I had a Cell C data contract through Autopage. After initial problems with the data loaded, the matter was resolved, and I had no problems with the billing of this account. In November however, the debit to my account was 12 times the normal amount, for the October billing. I promptly returned the debit, and requested Michelle Manley, my sales consultant, to look into the matter. In December I was debited with the October account, and another exorbitant amount for the November usage. After numerous attempts at resolving the matter, I received an itemized bill for the two months in question. NO response has been received to date, after my query was sent via e-mail, on 2 April 2015. My sales consultant eventually told me that they say that the billing is correct, and I have to pay! I have deciphered their itemized bill myself, and found that the problem started when they changed they cut-off date for billing, from the last day of the month, to the 28/29th of the month. Since no data gets carried forward to the next month, I utilize the remaining data in the last two days of the month. Is this too difficult to rectify, or can they not see how this happened? Service is non existent!