Mtn
Mtn blackberry Usage exorbitant airtime deductions

I am very unhappy with the mtn blackberry service. Whenever I load airtime it hardly lasts half an hour then the airtime is missing, when calling mtn the agents insists I have been using th Internet when in fact I cannot even use the Internet. Why does the airtime go missing? And why do the agents say it's Internet usage when this problem is country wide with bbm users? 90percent of mtn users has this problem. I am changing my network as soon as possible, as this form of abuse is not acceptable.