Question [Q] Making an Android VOIP app

Hi guys, I love XDA and there is alot of talent circling it so I wanted to bring these question to you guys.

How does one go about building a VOIP app for Android?, Are the calls free for the creator?(as in money is spent on development not on calls made my other users), How do those VOIP apps currently in the playstore allow you to register a new unused US phone number and how could I implement it to the application.

I appreciate you reading this and an answer if you can.

Btw I am asking because I want to bring really cheap calling through the use of VOIP to a 3rd world country as their current rates are outrageous.