Fongo powers free calling solutions for a number of brands. This forum includes information and tips on all of those offerings. Note that your specific services may vary based on the specific Fongo product that you're using.
Is there any chance of a build of the Fongo mobile app for Linux? Specifically, Raspian Linux? It would be great to be able to use Fongo on RaspberryPi devices, now that the latest RPi boards are all WiFi capable.
Doubt it. Too much cost for too little market. There are roundabout way to get Fongo on linux via an android emulation layer, but nothing direct, and not likely what you want.