Airtel — airtel number allocated to someone else

I am an airtel customer for more than 10 years and am out of india for sometime and i have taken the sim to london by activating international roaming. But however my sim is not seem to be working in london and then i requested my father to have a check in india while i m in london. But it was mentioned that my physical presence is required for some finger proof. I can back to india and when i enquired about my sim it was allocated to someone else. I hold this sim for more than 12 years now and have that number circulated all over that places like income tax, employer, banks and all institutions. My privacy is at a high risk now and i m really corncerned about this airtel answer of being a customer for more than 10 years. They should be checking with us by a letter before deciding on this as it s really critical. Kindly help me in this.