Incorrect billing

Reference EC-0W3A-2UJXGF, on the 18/2/2015 I took two tablets on a R99 per month top-up contract, I've subsequently been billed by Vodacom for data R6800 at the end of March and another R1000 odd rand for data at the end of April despite my contract being a top up one. Numerous telephone calls, written correspondence for them to rectify the problem has fallen on deaf ears. Incredible Connection have also written and sent emails to no avail. In the meantime I get emails to say that I must pay, the emails are computer generated so I can't respond. I need Vodacom to rectify the error.