AIRTIME GOES MISSING

I recharged with R30 MTN via internet banking. Note that whenever I recharge, i make sure the disable my mobile data on my phone, so after my recharge the airtime balance does not reflect the amount I've recharged with but a lesser amount. I contacted the call centre on 4 occasions. I was told that the "network" reserves a portion of the recharge amount for possible internet useage. So the steps to remedy the situation would be to switxh my phone off for 5 mins and back on and the agents assured me the issue would be resolved. It is 12 hours later a d the same issue is still not resolved. Can someone please explain why airtime gets reserved without proper notification? Why is my airtime missing???