Vodacom simcard miraculously using up data

On 16th March I received an SMS to say that my data bundle had been depleted - after there was over 400MB earlier that morning. Within a few minutes, my airtime disappeared too. I immediately thought an application was using the excessive data - so I deactivated all of the applications on cellular data. <br> <br> After uploading another 250MB data, I decided to turn on my cellular data for WhatsApp only - nothing else. By sending ONE message, containing two characters such as 'ok' - it used up 4MB data. This has continued and escalated to using up over 10MB data to send ONE message (the amount of data used varies - but doesn't stop).<br> <br> I've resorted to resetting the iPhone to factory settings. Nothing is installed, except the applications that are compulsory on the device. Wifi Assist is off - 3G reception only (as I've always had), NO applications open - data still being eating up on the line by Vodacom. <br> <br> I'm not subscribed to any services, I have no applications / e-mail / anything using the data, but I'm still having this deducted. I've tested my simcard in another iPhone - the exact same situation. <br> <br> This is totally unacceptable. I want answers!