Data usage being blamed on high bill

The past 2 months, my bill has been coming to R1500 plus..and that contract is only for R400. When I did query this at the pavilion branch,I was told that it's my data usage. Yet I'm getting data carried over ?The service at Pavillion is way to slow for me to waste my time to hear that they are trying to contact head office . And I did try contacting them directly , and their Line just disconnects after just giving them my details,oh and they do not even call back!I'm fed up and if I do not get a response I will block my debit from going through!