UPGRADE DONE WITHOUT MY KNOWLEDGE

I Have a Data contact with MTN which I only use on my Samsung Tablet, where I pay an affordable amount of R135. To my shock my bank account was debited in January with an amount well over R400!! I could not understand how this could be.. Yesterday 23.02.2016 I went into a MTN store to ask why they charge me this big amount all of a sudden? Because I haven't upgraded or nobody have contacted me to increase my amount paid. When I checked/dialed *141# I saw that MTN load me with minutes and sms's which I did not consent for. The Guy in store told me that my account was upgraded, which i haven't done and have no knowledge of. I still have my same device and i still only use it for data. He informed me that I now have to go and do an Affidavit on my on expense, drive around take leave to do this **** I did not even ask for and don't have time for. How can MTN be so careless and not even do the necessary security measures?! Can nothing be done the honest way these days?! Very Disappointed and concerned