Airtel — Airtel charging for some unknown international sms

Airtel has charged for some unknown international sms in my Feb 2021 bill. I have raised complaint multiple times in their app but this issue is not getting resolved fully. They just close this call without even talking to customer to understand the full issue. 2 sms each charge Rs.5 along with taxes. Message date mentioned in bill is 5th Jan 2021. This is happening frequently for past few years. Airtel is not taking any action in spite of multiple mails sent & escalated to nodal officer in past. They are not able to show any proof how this msg was sent. I get plain response this is due to usage of apple iphone. How airtel can charge customer without any sms history being registered in mobile & not sent by customer. Kindly look in to this issue for full refund. Also how to avoid this issue in future for apple iphone.