If people are interested in this type of thing for Android, I recommend checking out the RedPhone and TextSecure apps.
They're free to use, all the source code is GPLv3 on GitHub, and RedPhone already has global calling coverage. The apps have been translated into 15 languages, and in my experience they're really dead simple to use.
They're free to use, all the source code is GPLv3 on GitHub, and RedPhone already has global calling coverage. The apps have been translated into 15 languages, and in my experience they're really dead simple to use.