Vodacom cannot solve a data/airtime disappearance issue.

I spoke to Portia from your customer care department this afternoon re. the disappearance of airtime from my mother's pre-paid cell number ********** 650. She was incapable of telling me what was using her data, only that I need to change the settings. The data connection is already set off, there are no background apps running and WhatsApp only downloads chats over wifi and cellular, everything else only over wifi and OneDrive only downloads over wifi, yet my mother (a pensioner of 76, who cannot afford to keep buying airtime), her airtime money is disappearing to the extend that she now has the cell phone switched off and in the cupboard. I checked her phone this morning and checked the settings - her data connection is set off, her WhatsApp will only download over wifi, her OneDrive only over wifi, all background apps are set to off. Why is her airtime disappearing? Which apps are running so that I can check - she is now so scared to use her phone. She had no problems with the phone for the last 2 years and we bought her 30MB of data every month, which was enough to get her whatsapps downloaded if she was not at home in wifi range - now something is eating at her airtime. Please advise and contact me on my cell number ********** 366.

Kind regards Susanne Ableitinger