Telkom billing error

I recently checked my Telkom app which states I need to pay R1700+, this is strange as my contract comes to R539 pm, I recently rang up a spend limit bill of R281 which puts this month's bill at R820 which is fine and I will pay it but when I enquire about the remaining R900 I am met with excuses, how can my bill be higher next month than this month when I am paying for the excess. I phoned Telkom twice to which they stated they cannot generate the invoice as it is automatically generated on the 3rd of each month. This is completely unfair as everytime I enquire how they have gotten this sum they come up with some excuse or tell me to hold just to put down the phone. This is ********** service. How is it possible that nests month's bill is higher??