Money taken off prepaid account

Good afternoon. The lady that works for me had money going off her prepaid account without making any calls or sms's. She called MTN and asked them to tell her why that happened. They said that she should load money on again this morning and not make any calls or sms's and see what happens. This morning she put on R10.. a little while later she was left with R0. She called the call center and they said it is impossible to trace where the money went. How is that possible?? MTN should have record of why money get deducted off prepaid accounts. If someone could please urgently let us know what is going on and why the money is going off her account and STOP this from happening. She cannot use her phone at all until resolved.