Account in dispute

I was charged with data charges to the value of almost R30000 for the months of July, August and September. This account has never exceeded R700-R800 a month and now it's R30000. Although this account is in my name this is my moms phone. Ive had data validation done but no one at Mtn can tell me with cer*****y what happened. To clarify my mom is 62, she has almost no apps on her phone nor does she know how to perform software updates. Logical sense tells me it's impossible to use that amount of data. I have spoken to numerous people including call center agents, and managers..nobody bothers phoning me back. This is terrible service from Mtn. I have been a loyal customer for 15 years and this is the service I receive. As a matter of interest I know of two customers who are Vodacom clients where the same thing happened and Vodacom actually was able to say min what the data was used on as well as clearing their accounts. Kedibone Mkwa**** promised to change the account to a top up plan, but hasn't even phones to confirm that it's been done. I've had a conference call together with ABSA and Mtn, spoke to manager Sam Mdunge at Mtn who would have also phoned me back but failed to do so