Not giving all information

I have been with Telkom for about 2 years now and never had an issue until yesterday when i received an sms stating that my account will be debited with R 1300-00 end of November 2015, this is all good but my contract is only R 399-00 per month. So I contacted them and the told me the following that because I upgraded after a certain date I now have to pay for the previous month also\Pro rata"which I understand completely but the thing is I called their accounts department last month and they confirmed with me that because I upgraded after this certain date i will not have to pay for the month the upgrade occurred in"