Airtime disappearing

I called MTN last week to report that my airtime is diminishing by 3 cents they told me that everytime I go onto the internet I am charged 3 cents. I told them that I pay for uncapped BIS for my phone so no money should be deducted. They \escalated"my problem but I did not hear from them and the deductions stopped. It has started up again last night from the time I checked my balance I have lost R1"