Daniel Chote (a New Zealander now living in the USA) has built a walkie talkie for his kids out of a Raspberry Pi, some small electronic parts and a 3D case. When you press the button, you can use the internal USB speakerphone to talk to other participants. The software backend is Mumble which means you could use your “talkiepi” to talk to anyone running a Mumble client – even a PC, Mac or other Linux machine. He has extensively documented his build here and has written an install guide here. It’s a great project from someone who cares as much about what the device looks like as how it works!