billing problem

Mtn has been charging me hefty amount of money on my contracts first i have a top up 100 account for 239 and a 500 mb data plan for 249 november statement is 601.00 and december is 1250.71 and the sim is not always on ,as i misplaced it but they tell me my data is always on so i dont understand all of this ths is **************** firstly when i buy data bundles and i have airtime on my phone they always tend to finish my airtime first before the data bundles are even finished mtn is robbng me of my own hard worked money i dont know what else to do everthing started well as they were deducting the right amount and it changed all of a sudden and it seems im sharing this number with someone which means im paying for someone i dont even know here i want all this sorted as im tired of this ...im suprised how can they bill so much money to a sim card that is off ? I need a full statement of where all this charges comes from