Telkom Billing for service not used

I have a telkom mobile house phone that I had with telkom for years for my mum who is 81 years old. I opted for fixed unlimited usage with telkom of R671 per month. This month I received a bill of 2100 rand on querying the amount they claim that there was an airtime transfer and once off data transfer done to a number that not even available ***. I or my mum did not even know you can do that with that account. Calling into telkom and they say that cannot help me as the transfer was done. Totally unacceptable