Overcharged and frustrated

Mtn has been invoicing me for data I have not used and when I call to log my query or dispute they closed my calls within minutes of dropping the call. My cellphone data usage shows that I have used 1.6g of data yet on the invoice they charge me for 6g and my cell is mostly connected to the wifi at home or via Hotspot from another cellphone. I don't have any active app and use the phone only for WhatsApp calls and sms's.