MNT is *****ing from my elderly mother

My elderly mother has a phone contract with MNT, but they are forcing her to by double each month. She usually pay her account in the first days of the month but around the 15th they cut her phone of line. She went to quiry it at Oudtshoorn MTN branch but they told her if she pay her self then she will pay double a month but if it's a debit order she pays only once a month... How in the hell could it work like that, cause if se refuse a debit order and pays it manually her self se has to pay almost R600 for R250 worth of phone... Something is totally not right. Please respond se can't afford to pay twice in a month which is anyway absurd never heard of something like that and secondly she is elderly she needs her phone to be on air.