U choose flexi sham

I took out a u choose flexi for my son a few years ago. I chose this contract because spending is supposed to be controlled. If there is no airtime you cannot buy bundles etc.

On the 1 Jan 2020 I logged into my app an saw that there were talk minutes and data on his profile that I did not load for him. Details werent available so I thought it was a mistake.

I received my invoice on 10 Jan and have a whopping R2300 Bill. There are additional bundles on this contract. Not loaded by me but resulting in additional charges which is not suppose to be possible on this type of contract. I called vodacom and i was advised that this resulted from someone calling *135# from my sons number and loading. I strongly disagreed because even if this happened, this contract is not supposed to allow additional charged.

I need answers as the app did not show the additional costs during DECEMBER 2019 either.