this is a very simple trick that might fetch u 200mb free on your airtel sim. some sim might get only 100mb sha.
Here is the process:
1. first migrate to airtel club 10 paddi plan by dialing the short code *166#
2. dial *141*13*200# for free 200mb or *141*13*100# for 100mb.
good luck and don't forget to share this topic if it works for u.