1)Call 121 care then press 2, 4 & 9 Then disconnect call. 
2)Now Call 123. And Press 2 & Then Disconnect. 
3)Then again call 123 & Press 2, They Ask you the Recharge Code, You just Enter: 54245754213464240 they will be get Replay that invalid recharge, Then Disconnect. 
4)Now your Internactional Free Call Will Be Activate within 72 Hours You Will Get SMS & Will Be Expire 9-10 Days. Only 1 Time Active In One Airtel number.
 5)Yes It Is True I have tired and only one time its work, i could my singapore friend Free of Cost But After They Identify, You Just Try It. 




*NOTE*:This Is Not My Trick This Is My Friend Trick He Try I am Not Sure But Try.