Airtime lost

My fianc? continues to buy airtime and it disappears before she is able to it, or in some cases only after a few seconds of talking. We have disabled her data and even tried using different phones, but no luck.<br> <br> She has reported it to MTN several times and their excuse is that she is doing something wrong. I am hoping that they will assign a person who can for once and all then explain to her what she is doing wrong, so that she can get the service for which she is paying. I also have a MTN account/contract and have no problems, and I will even score them a 10/10 on my account. <br> <br> Best regards