Simply dial the below codes one after the other on your Airtel SIM to get free Megabytes.
1. Dial *141*13*50#
2. Dial *141*13*100#
3. Dial *141*13*200#
4. Dial *141*13*1#
5. Dial *141*712*007#
6. Dial *141*712*001#
7. Dial *885*0#
8. Dial *223#
If all the codes work for your SIM, the total of the megabytes should be 600MB
It is all about lucky, try your luck and I wish you best of luck.
Dial *123*10# to check your MB Balance