Missing airtime.

On the 1ste of February I loaded R240 worth of airtime and on the 4th I got a SMS notifying me the airtime has been depleted. I only made a hand full of calls so I requested feedback from MTN, after 20 back and forth no answers were given, I provided them with my spending bill and they provided me with my call history proving I didn't spend the airtime yet they are incapable of telling me where is my airtime. All the emails just give generic answers that make me think they don't really even read the complaints. *** is really useless. How should I continue doing business with MTN if they lose my airtime and then provide zero feedback on the matter.