Cell C impossible to sort out their fault

My daughter immigrated and got cancellation figures, paid it in to have her CellC account closed. Cell C didn't close the account when they were supposed to. They dated the closing date for a year further. They only closed it months later and they expect her to pay for their fault. They now handed her over to SSDA, attorneys who phone me every day. Worst of all is they charge for every phone call but they do not want to speak to me although I have a power of attorney, registered in court, to deal with their affairs as they immigrated. I phoned CellC numerously, sent the power of attorney but they refuse to speak to me.