Contract conditions being Bridged

i have signed a contract with Telkom for a DATA deal and one sim car got free minutes, i have put the sim card in my sons Tablet as i knew its only data on it..Telkom has now put a R500 recurring wallet for phone calls without my permission ...i want to know who authorised this to go through on my data deal package and how is this going to be rectified if their is no signed agreement by myself for this R500 recurring wallet then Telkom must reverse the charges off my Telkom account.