If U want to Set 'unreachable' or 'out coverage area' in your airtel

First Keep More Than 1rupee in Your 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 9895012345
And divert box
enter this
989501234
Remove last number of ur mobile number.
It is very useful.

The other way is:
Just remove ur mobile's battery without switching off mobile,atleast 30 minutes caller will here unreachable sound.