Credit Refused to pensioner

Since my mother is a pensioner and aged 69, I handle all her correspondence, mail, internet banking etc. She was notified by MTN that she is due for an upgrade on her cell phone. Upon receipt of the invoice, she noticed that she has a credit to the amount of R8357 on her account. My mother phoned MTN and asked for a refund but MTN refuses and claims they will allocate it to the new contract. At this age my mother is not interested in any form of commitment and does NOT want to renew her contract. I phoned MTN to try and solve the matter but since I am not the account holder, they refused to discuss any details, yet the MTN invoice is sent to my email and all MTN related correspondence is also referred to myself. I feel it is utterly disgusting that MTN which is a big institution refuses to refund a pensioner on monies owed to her. Most certainly this is not lawfull?