Billed R8000 + for data in 15 days - no proof

I have had two cell phone contracts with Vodacom for more than 10 years and last year renewed the contract with a fast-talking salesman who sold me a really dud contract. What I did not know is that the new contract on the one phone did not have a data bundle. I gave the phone to my son to use and within 15 days got a message from Vodacom that there was \unusual"activity on my account. I phoned in and discovered that the data usage was R2050 for April and R3700 for May. They could not give me how much data was used and when as I did not have itemised billing. So I have to pay the full amount just accepting that is what I owe. I asked them to suspend the number and if I could pay the amount off over a few months - their reply \""no"