Airtime disappers and Landie dropped phone in my e

On the 11th of March I checked my airtime and my balance was showing R160.00 the following day when I wanted to make a call it said insufficient funds, my airtime just disappeared so on the 12/03/2015 I bought for R30.00 and on the 13/03/2015 I bought for R60.00 a few days later my account was showing R180.00 supposedly I got my airtime back, this month the same thing is happening, so I called MTN and spoke with Landie and explained to her what's happening and she advice me it is internet usage and calls I made, which I don't understand, I explained to her what happened last month and she got frustrated and drop the phone in my ear, this is unacceptable I am paying my contract every month and this is the service I am getting, I want my airtime back, same as last month and this thing must stop immediately.