Airtel — Inability to make a call or receive one

For some reason i was not able to make calls from my phone. So I walked into a nearby airtel store where I was told that the card was old and hence they replaced my card. It did not solve the problem. The guy in shop fiddled with my handset and he could not do anything. I went to another store, however he could not figure it out as well. The issue is my new sim works in other phone, but not in my phone. At the same time my phone accepts another airtel sim and allows to make a call, but not this one. I have thus not been using the phone for last month and my 400 rupees have been a waste largely. I went to the store again on26.10.20. They changed my sim again. It is still not working. I went on 27th, and a guy tried to help genuinely. He said my calls were barred and he was trying to reset the same. No relief so far.
I tried calling on the customer service numbers which are useless. There is no option to talk to a real person . There is NO way that I can get my problem sorted out and I am tired now. The app dignostics says your phone is 4G and upgrade the sim. Your shop said it had an issue with VOLTE. Now if it was so, why was it working fine for quite some time? If my issue is not solved in next four days, I will be forced to switch to other provider.