Airtime and data disappear

So I generally buy airtime and data as and when I need it.. sometimes R30 other times R60 at a time and have never had airtime literally disappear until now. You are I went to Namibia for business and so I did something I had never done ... I purchased R1000 airtime because I know roaming can be expensive and I wanted to be prepared in case of an emergency! Well long story short, I never used a cent while in Namibia as I deactivated the roaming and purchased a local sim card. Anyway when I got back I had R998 and 1.10GB of data available. On Monday, 25 September I still had approximately R970 and 1GB available ... and that's were things go haywire because even although my mobile data is deactivated 80% of the day and the only thing I have used when connected to my mobile data is Facebook and whatsap, exactly 3 days later I am left with R0 airtime and 0 data. So I call the call centre who tells me I have a high internet usage and therefore used up my airtime! How??? #MTN suck and are the biggest ******!!! The fact that I am disputing this means nothing to them. They just keep giving the same generic answer... I want my airtime back! Do yourself a HUGE favour and stay away from MTN!!!