Airtime used up with no reasonable explanation

On Saturday the 21st of November my prepaid account was topped up via cellphone banking but within a few seconds i was already R3 short. I then switched off my mobile data and accessed my bank account but couldn't continue with the transaction. upon checking my balance i was shocked to see i was only left with R0.10 worth of airtime. I then called the customer care to find out what happened and the consultant said it was because I had been on the internet which in actual fact i wasn't. I asked how this could be possible because i had switched off my mobile data and only accessed my bank account. then she said they charge a maximum of R1.88 for cell phone banking but they took all my airtime. the consultant couldn't give a reasonable reason as to why this happened and kept insisting that i was on the internet. I'm very disappointed to say the least and i certainly will be changing networks.