Airtel — every month rs 5 is been charged for an international sms which is not sent by me

From last 6 months airtel is charging me rs.5 to my bill every month for an international sms to[protected] which i haven’t sent and i do not know this number nor i have sent any sms to that number! When i spoke to airtel customer care they says sms was sent from my mobile. I tweeted about this and after my tweet airtel customer care contacted me and said i won’t happen again in future and they gave rs 50 wave off in my next bill. But again i was charged for this month and this it’s rs 128 (For late fee, tax and sms charges) when again i spiked to customer care they says they can’t help me with this because last month i was given rs 50 wave off. I mean this is ridiculous from airtel give rs 50 wave off and charge rs 128 extra in next bill. It’s a pure harassment from airtel.