[Q] How to make phone appear out of netwok topic

Well what I basically want to do is to make my phone say "out of network" or "User busy on another call" when someone specific calls me. There are plenty of apps out there on play-store to blacklist calls but what they basically do is that they cut the call with the caller listening a minor ring, means he comes to know that his calls is intentionally being ignored.. I also know there is trick of removing battery while phone is on but in this way I can loose my other important calls..!! I've come across an app in Intex Aqua series of mobiles named "Firewall". This app does what I want to do but when I tried to install this app on my Xperia SP it doesn't get installed might be there are proprietary restrictions. So does anybody knows a trick how to port that app for Xperia or could do this then it will be of great help.. Thanks