Startup Idea : Biggest Phone Company In The world

I'm think of creating a VOIP phone system where anyone in the world can be able to sign up for a number and then add that number to any voip client or device, then make and receive calls over voip. I believe that phone calls to anywhere in the world should be free and the main goal of this idea is to make it as simple and convenient as possible. You sign up for a number and then you add that number to your VOIP phone and once your friends and family do the same, you can then call them free forever as long as you both have internet access. I know a lot of service offer similar concept, but I don't think any is just focused on giving you number via voip and then give you the ability to add it to any voip device or client.