App making, I have an idea and I need a little help

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This is not an original idea and that is why I am not afraid to post it right here for the world to see. It is something that is flood all around us but if I could pull it off, my geographic location would make it unique. The idea is about voip (voice over internet protocol) calls. I wanted to create an app that could call landline and other phones, not just an app that can call another app but it needs to be able to call phones. Why would it be unique? It would be unique because I am not interested in international calls, I am just interested in calls within my country. All of the apps so far that does it and do it for free are based mainly in america. Meaning you can only call in the america free. I would like to take that same principle and make it so that I can only call my third world country free. It would be made for my country and I would like it to be free for everyone in attempt to lower the costs of making phone calls when you have an internet connect. I always wondered how magic jack did it or google hangouts. How did they do it to make it free in their countries, and why does it cost more to call international? I really wish there was someone I could talk to. The more I research the matter the more i spend time not moving forward. Any help would be fantastic