not informing me correctly

I have a cellphone contract with vodacom. I placed a voluntary limit of R50 on my contract. To My surprise, when I received my statement this month, from a R200 month bill it went up to almost R700. I phoned vodacom and spoke Tebogo at the call centre. He told me that it is my data usage. So Told him that i have a voluntary limit on and that is normally suppose to switch my phone off. He said well vodacom can not do that. So i said that it ia not treating rhe customer fairly. How on earth is that possible that from a R200 contract im paying 700