Data went M.I.A

Here we go again vodacom, i've laid countless complains, and i'm really getting ticked off now. Today somehow I've used 250 mb with just whatsapp messages??? Not even doenloading photos or anything, their data bundles are so expensive but I need data I would like to know what exactly on my phone is using so much data when I have no background apps running?? Sometimes I switch my data off but yet my data dissappears, I tried contacting the call centre now and they are recieving high volumes of calls...why not employ more call centre agents seeing as vodacom is the most expensive service provider there is, and should be the best might I add they infact have the best coverage but the worste customer service...We need to fix this problem please Urgently!!