This works for me. But please note it is not the same as visual voice mail what you get in USA.
It works the classical way.
To activate voice mail in Airtel (Karnataka region).
Dial
Step1 dial *321*671#
This will activate voicemail in your Airtel phone
Or
Send SMS: START VMS to 52555
Step2 dial 52555
Follow the instructions and enter your new 4 digit pin
To activate call forwarding to voice mail during busy dial *67*9840011999#
If busy
Check Status: *#67*11#
Activate: *67*9840011999*11#
Deactivate: #67*11#
if no answer
Check Status: *#61*11#
Activate: *61*9840011999*11*[sec]# choose anything 5 to 30 secs
Deactivate: #61*11#
If unreachable
Check Status: *#62*11#
Activate: *62*9840011999*11#
Deactivate: #62*11#