If U want to Set 'unreachable' or 'out coverage area' in your airtel
First Keep More Than 1rupee inYour Mobile
then goto diverting option.
Then go2 Setting
Then select cal divert option
then activate other number
then type that column
Remove ur last number and put it.
Eg:- your number is 8129970907
And divert box
enter this
812997090
Remove last number of ur mobile number.
It is very useful.
The other way is:
Just remove ur mobile's battery without switching offmobile,atleast 30 minutes caller will here unreachable sound.