BAD SERVICE

I Recently upgraged my contract with mtn and was told that my account would be capped, meaning that i pay one amount every month and should the airtime/data be finished, I would then need to buy airtime/data and not get billed for it. This however is not the case as i was debited R1667 at the end of April and for May i need to pay R1500. This is not what i agreed to and the consultant at the store is not able to assist me all she says is that she has loaded a query and not getting through to customer services. i need confirmation that my account would be capped as this is what i signed up for, not to be billed and me not knowing what amount would be debited from my account. Also, i get sms's saying thank for data bundles i bought and i have not purchased anything, so who's data am i paying for.