MTN AIRTIME DISAPPEARS

I have tried for ages to find out why R10 airtime on the simplest mobile phone has the balance removed/taken/disappears every single month after 21 days. The R10 does not get used up. I have tried to get a response from MTN and Icasa. I am told there were "international smses" made! This is untrue. The phone is as basic as can be. It is used to make and receive a few local calls to and from the same local mobile number every month. The user is unable to send smses. Why is the balance of the airtime disappearing? Why is it not carried over until it is used up. If smses were sent as I have been advised, I asked for a printout but no response. I await a response from someone.