MTN poor service

I recently signed a new contract with MTN for which I receive 200 minutes and 1.5 gig of data a month which I receive on the 17th of each month. During the first month of this contract i noticed that my data lasted about 3 days? and my minutes disappeared quicker than anticipated. When I received my data and minutes on 17 December I rest my phone to show 0 data used and minutes used. I also switched off mobile data and only use my home WiFi for data. Within a couple of days my data was gone. To date I have used 47 minutes out going calls but my minutes left is 11. According to my calculations 142 minutes have disappeared. I have been to the MTN outlet on 3 occassions and they cannot help and their supervisors are missing in action?