1) Send MRP FULLTT to 51619 for available full talk-time offers...
2) Send MRP TRF to 51619 for available tariff offers...
3) Send MRP SMS to 51619 for available sms packs...
4) Send MRP GPRS to 51619 for available gprs packs...
6) Send 3G to 121 for activating 3g or change 3g data plan...
7) Send START to 121/1210 to start any service...
8) Send STOP to 121/1210 to stop any service...
9) Dial *555# for daily and monthly sms packs...
10) Dial *222# for special five offers...
11) Dial *141# for airtel gift service...
12) Dial *325# for facebook fontwish service
13) Dial *121# for my airtel my offers
14) Send FUN to 54321 to activate airtel live (not working on new sims now)...

15) Send LIVE to 52567 for airtel live settings...
16) Dial *123*7# or *123*10# for checking gprs data balance
17) Dial *125*5# for daily sms count...
18) Dial *123# for balance n validity...
19) Dial *567# for mobile office activation n deactivation...
20) Dial 121 to call CC
21) Dial 198 to lodge any type of complaints...
22) Dial 123 for recharging your account...
23) Dial *566# for musstbuy service...
24) Dial 198p2p1p4p2 for direct call to cc...
25) Call 1909 to activate or deactivate DND service...
26) Dial *515# for twitter service.
27) Dial *123*11# for checking 3g data balance

Hope this is useful :)