Despicable Treatment of Senior Citizens

My mother retired and changed her banking details. She updated all her creditors successfully except for Netstar. She tried to change it multiple times online and even over the phone. Her account has been handed over now and is somehow owing R 5300.00. I don't know how this is possible when she has made manual payments of about R 2100.00 and the account is only R 400 a month. Owing R5300.00 would mean she hasn't paid consistently for over a year - how is that even possible? When I try to call the legal department to assist her on this matter, there is no answer. This is my last resort.