Someone explain how you charge your data usage!!!!

I've been having problems lately with my data usage on MTN. So I have both data and airtime but every time I use my Apps My airtime balance gets deducted and not the data balance. I'm sitting here with data which I buy monthly not being deducted and have MY airtime balance deducted. Called your call centre countless times to resolve the issue, I hold for 25 minutes then I get cut off, EVERYTIME. This is absurd. Now I have to buy airtime and data while I still have data on my phone. RIDICULOUS