MTN Invoicing

I have just taken a contract with MTN, I opted for a deal which was R689 per month, to my suprise my first invoice arrives R1160????? How the hell do you get to that amount, I check my mobi app and it says my invoice for July which will be due 1 August is R989 again NOT ACCEPTABLE as that is not the contract I signed. I need feedback NOW not within 72 hours.