Incorrect Invoice

I have been a telkom client for years and was happy with the service until recently on the 16/10 I bought a loop which started all my problems...I have never been behind with my payments as I pay monthly however due to a promise to pay option my whole invoice got affected...I was requested to pay R1250 for the month of 1 November which I did however I was not aware the invoice does not include promise to pay when I asked for it to be removed since I payed they say said its impossible question is I used to pay R589 monthly without skipping now I pay R1250 how is it possible that I owe R533 please make me understand as I have never skipped any payment