Vodafone — VI illegally overcharging me

Vi (Vodafone) has been overcharging me for the past two months by almost Rs 1000. The bills mention international roaming charges but my phone data has been switched off and no call or sms were sent. I was never informed of any additional charges and didn't subscribed to any. I was only told that in case I make a call or send an sms a Rs 500 roaming pack will be activated.
Vi's customer care sends me emails repeating that the bill us genuine but they have refused to give me any explanation as to how and why there were additional charges on my bill although I did not use my phone at all and the data was switched off.

My relationship number at Vi:[protected]

I want a full refund of the amount Vi has overcharged me for.