Unagreed spend limit

Hi there I'm not happy about my August billing. I only downloaded my telkom app month end and I nearly fainted when I saw that I'm owing R599. From my knowledge I'm charged R99 for my contract monthly.i called telkom and I was told about spend limit of R500 that I know nothing about.I don't remember agreeing to that when I agreed to another 24 month contract of 20gig data. To me it was a saving because I'm from R149 to R99. I mean how on earth am I going to get that much money to pay in one month. For the past 24 months I never had that R500 limit and all of a sudden I have it how I don't know. Please do something about this because honestly I'm not going to pay that money because I don't have. My contract is 20gig which is 10 gig day and 10 gig night and when it gets finish before month end I top up or I wait till month end. Can you please resolve this urgently. Number in question *** and case number ***1.