How is her faulty?

My sister got a data contarct from MTN, gave then her bank details to deduct money but they didn't she wasn't aware.. today she went to do a sim swap and she was told that she had to pay more than R1000 because she owes MTN. I mean that's all she had on her bank account and now it's gone because she wanted to her phone. Someone ****ed up I'm MTN now my sisters last cent is gone. They couldn't even do payment plan...