Steps-:
1) At First keep balance 1rs. to 2rs.
2) Dial *121# select Data services.
Click on Re 1 store.
3) Now you will recieve message external link.
4) Just try to open the link 4,5 times with default settings.
5) But it will not open. now call customer care and say "am not able to browse in my airtel sim".
6) They will check your problem and unblock your sim by take an comlaint about browsing issue.
7) Now they will unblock your sim,
Now enjoy free internet again in your airtel sim.