Airtel — being called by unknown numbers

I already have a bsnl number and i took a airtel sim 2 months back. Today 18th april 2018, i got a call from an unknown number from mumbai. That person is repeatedly calling me saying that i had called him several times and disturbed him when he was at work. He saying that he will file a case.
What nonsense is this??
I don't even know that person nor i have called him ever or messaged and i am getting this stupid threat for not doing anything.
I had been getting few unknown calls when i had bought the sim.
Please note
Airtel can't manage the issue of new sims
Please make sure that there aren't two sims having the same number
When issuing a sim to a new person please ensure that the previous connections are cut and the person cannot call from the old sim anymore
This is totally creepy