How can you get free balance in airtel with out any cost?

This is the sms which i received on my no.

"Now add 10 Rs. Balance to your Airtel prepaid account anytime, anywhere.

Just Send LOAN to 121 [ tollfree ]. This 10 Rs. + 1 rs processing fee will be

deduted after 48 hours from your account"

So if there is an emergency to get some Talktime then

dont penic just send LOAN to 121