Being ****** by MTN

Since December 2014, MTN have been robbing me blindly of airtime! I have been spending between R500 and R700 PER MONTH on loading airtime but have honestly only been using about R100 MAX! I have no clue where to and how the rest is disappearing but it literally happening overnight, or even in the space of an hour on some days. The only apps that I use are Whatsapp and FB Mobile Lite but I have purchased data for these which does decrease in value so I know that the apps are using my data and not my cash airtime. I have tried using three different phones, deleted apps, etc but this has made no difference to MTN somehow accessing and robbing me of airtime on a DAILY basis. I am in debt because of the amount that I spend monthly on airtime purchases and I absolutely disgusted at the so-called \customer service"from MTN. I have heard lately that several of my colleagues are being ****** of MTN airtime in much the same manner...when will this come to an end?!"""