DATA DISAPPEARS

My son's data disappeared and started eating his airtime up (AGAIN). He was unaware that his data had run out. Sms's don't go to his number ***) notifying him that his data has run out, they instead go to another number ***) which is on the same account number. We have tried to change this on numerous occasions but without any luck. The number the sms's go to is not always switched on. This is a major concern. Please rectify this issue immediately!