R800 mystery addition to bill

My mom has worked in the UAE for the past 2 years and for the past 2 years has taken her phone there and she has never been charged extra as long as she didn't accept or make calls with her phone. She is on a top-up contract with a R0 limit. At the end of November she got a bill with an additional R800 on it. It showed the R800 as International Roaming where it also showed 0 min of calls had been made. That's R800 for NO TIME ON THE PHONE! As I have power of attorney and we gave the relevant documents to MTN before she left I went to query the R800. After a huge fight because they had somehow lost that information I showed my ID and the relevant PoA documents and they agreed to help. They said that she was being charged because when people call and it goes to voicemail she gets charged. Why this only started happening in November is a mystery and how they can say they won't charge her because she is on Top Up and then end up charging R800 extra is not acceptable. Anyway, she decided to pay the money and deactivate voicemail. In December we were still charged R500 for 0 minutes of International Roaming with 0 minutes of call time. HELP!!