Airtime decreasing not bundles

I had an amount of airtime on my phone as well as 328 mb of data. After a while I received an sms that I have run out of data. When checking my balance I noticed that I still have data on my bundle but my airtime balance was used. This happened a while ago, but I just loaded a data bundle and now again. I tried to contact MTN on 808 for several times from my phone. It gets cut off EVERY TIME!!! I then phoned them from a Vodacom phone. After holding for about 15 minutes I got through to a consultant. Unfortunately the call got dropped. By this time I have spend 23 minutes on the phone. Note that it is a charged call because you simply can't get through when you phone them from a MTN number!!! The consultant didn't even bother to phone me back although she was busy with my query. MTN's service really SUCKS!!!