Airtel — airtel 4g 128k sim not working

Address: Varanasi, Uttar Pradesh, 221010

Website: Airtel.in

I had upgraded to airtel 4g about more than an year ago. Till now i never got any issues but suddenly the network was lost on my phone and the phone said, "sim card not ready / emergency calls only", i inserted my airtel 4g sim to a 2g handset and found the signals and everything working alright. The problem was only with 4g/3g handsets (I tried inserting my sim to other handsets too, but the problem was same) where the sim wasn't working. The sim also made my phone to restart and irresponsive.

I visited the nearest airtel store. They gave me a sim and asked me to send an sms to 121 with xxxxxxxxu and then reply with 1. I did that but yet the signal in the old sim didn't got disconnected. I'm using the sim in my 2g handset. I talked to airtel customer service and they said i can't do anything by sending that sms because i already have upgraded to 4g. I'm confused, here the airtel store guys say something else and customer service fellows say something else..