Airtime dissappear with MTN

I did a test now bought 5rand airtime to see if it will disappear ....and **** it disappeared...tried to call customer care 136....no option to speak to a consultant...would like to know why does my airtime simply disappear every time....and dont tell me about your mtn video or promotions for R7 per day....im not interrested and even when one unsubcribe for something u never subscribed in the first place ....airtime still disappear....WHY