BILLING NIGHTMARE

I am traumatized!!!! Eversince I took mtn contract my days are spent calling umyour czmall Centre with no help. I took a contract (phone and 1gigSim) for R320 a month but for the past 3 months I've been debited and paying R1200 or more. Even when Im told my limit is set at zero and I no longer even use the Sim card. Im even scared to use it die to charges but even now they saying I've been buying airtime worth R500 and purchasing 20 gigs. I have a separate router at home and wouldnt sabotage my self like that. I NEED ANSWERS MTN. I DON'T KNOW WHAT SORT OF **** YOU ARE RUNNING OR YOU HAVE EASY ACCESS FOR ******** TO USE OUR DETAILS. I NEED A REPLY OR THIS MATTER IS GOING FAR!!!!