Vodacom debiting more money for their mistakes

When my mother subscribed to vodacom for a new contract, an additional line was added on her account without her authority and more money was deducted from her account and that additional phone was never delivered to her either. we noticed on the statements that additional money is going out then we contacted them to rectify, they did acknowledge that and they said they will investigate. Vodacom came back and sent us up and down and not giving us answers and did not fix the issue. i have been in contact with them numerous times but they only refunded some of the money but then again decided to take more that what they refunded. please assist