Telkom varying data contract conditions at the push of the button.

I currently have a data contract with Telkom with account number: *** and cell number: ***. My Contract conditions are such that I pay R 207.30 for 20Gig anytime and 20Gig night owl. I discovered just browsing through my bank statements that actually Telkom are charging me a variable amount monthly and the highest they billed me is R 2 031.78. Upon enquiry I was told that the simcard once it reaches the limit of 20Gig whether you want to continue. if you press yes..more data is made available just at the press of a button. This particular router is used by my 2 sons and they are very gullible. Is this type of contract even National Credit Act compliant. This contracts gives teenagers the power to access more credit without even blinking. For me access more limit on my bank credit card or store credit they are specific questions asked based on your income and expenditure and records required to ensure affordability. Why is this Telkom contract not complying to same conditions. Looking at my bills for the past 5 months commencing December 2020 usage, I have paid an extra R 5 140.48 beyond what I agreed with on my original Contract. This accounts for an average of R 1 028 extra monthly that I did not budget for and its going to Telkom without my authorization as the account holder.