Airtel backs with a free 4G data just by giving a missed call to a number. This offer is only valid for selected cities only. Perfectly working on Telangana, Andhra Pradesh, Maharastra, Delhi, Kolkatta, Chennai, Bangalore, Kerala, Haryana etc...you can try this in your state and comment below.
Lets See How to Get Free 4G Data On Airtel 4G Sim :
First Switch your Mobile and Airtel Sim to 4G Compatibility
Dial 52122 with your Airtel Number
It automatically Disconnect after dialing
Now you will get One Messege Called You Have Got "1GB 4G Data Valid Upto 90Days"
You can check your Free 4G Data Bal just by dialing *121*2#
Remember : Connect your Network to 4G and Use the Data