My airtime being eaten up for the internet.

On 23 October 2017 I put in R29 airtime and bought the R12 whatsapp data for 30 days. The balance was R17 and received 25 free MB and 25 free minutes. I used those MBs during the day to read messages on Facebook and twitter and the balance left was 16MB and R17 airtime. At night I just read messages on Facebook and twitter that's all, unfortunately I didn't check my airtime before I sleep. On 24 October at around 07:30 I checked the balances and i found zeros both on airtime and MBs. i called Cell C and they told me my airtime was used up by the internet. When I asked what internet can take R17 since i don't believe that even if I mistakenly went to internet it can't take R17 for one or two seconds. So i need Cell C to give me the website I visited and for how many minutes for them to charge me R17. It it is the four time this happens to me, even wife has experienced this several times. I need Cell C to tell how they charge internet(is it seconds or minutes) and how much they charge. If a customer hasn't got data, how much they charge from the customer's airtime?