Airtime disappearing and reappearing

My mother's MTN airtime keeps on disappearing and reappearing on her prepaid account. She is a pensioner and can not afford to be without airtime at any given time. This has been going on for months. Why is this happening? How can a service provider take away prepaid airtime and leave a pensioner stranded??? My mother is currently traveling and can not rely on using her phone??!! This is very stressful for anybody let alone a pensioner. This is totally unacceptable and is nothing less than *****ing. We load airtime for her which shows on her balance, but when she wants to make a call the balance is R0.00 and 0MB.

I see various people are complaining about the same issue? Is MTN doing anything to resolve this issue?