Airtime disappearing

MTN sort billing problems please. It seems Nashua Customers got the raw deal when account were transferred to MTN. MTN should be investigating why previous Nashua Mobile customers complains a lot about billing but why should they do that it's more money for them. Now my new complain, why my airtime keeps on disappearing. The month of Jan R200 disappeared when I called consultant said backround apps are running, she transferred me to technical department which I hold till the line cuts off. Then Feb month I didn't bother checking. The month of March 2015-03-07 airtime disappered again or maybe I do not know how your billing system works. I'm on anytime R350 which means I should get R350 airtime correct me if I'm wrong but that's how Nashua used to bill me. I usually get airtime on the second of every month. I've login on the internet to check my unbilled calls guess what I've only used R 68.77 from 2015-03 to date. So where is my R281 worth of airtime. Since MTN charges everything separately data,sms,etc... where did my airtime went. And don't tell me about backround apps cause nothing is running.