There is a app called BlackList. That is meant for thing ur looking. It acts like a screening utility giving a busy tone to the selected phone nos.
I wanna keep my phone busy to all nos., not only for a specific number.
In the same "Black List" software,there is a provision called white list whereby you can choose phone number(s)for allowing calls,For all other numbers it will be like 'busy response'.
NIce software !
You can use Smart Call manager. Recored the beep sound that is played for a busy number and use it to automatically answer all the calls.
On Airtel, even if you reject a call, the network gives message that the dialled number is busy. So if on Airtel, you can just ask it to reject each call automatically and the caller will think your no. is busy