Airtel — my number has been allocated to somebody else

My number has been allocated to someone else without prior knowledge to me. Some of my bank accounts, my insurance info is connected to that number and now it’s all in the air, in the hands of someone i don’t know.
Why have i not been informed about this in not even a single email. Airtel, what was so tough in reaching to your customer? I have been airtel’s customer since more 10 years and this is what it melts down to.