Hackaday mentioned some time ago an interesting Arduino cellphone. The fact that you can build a cellphone around an Arduino is pretty neat: An Average Joe can build this thing with a minimum or background knowledge and without breaking the bank. Wow.
The components include and Arduino Uno, GPRS shield from Seeed Studios, a TFT touch screen, Lithium battery and charging circuit, and a few other bobbles. All of it is mounted inside of a 3D printed case. You can get all of the Arduino Phone code from the Github.
This kind of hacks really show that cell phone phone business have evolved to point where even relatively small companies can start to make their own phones if they want. Nowadays when you base your design on some readily available GSM/GPRS/3G communications module, building the phone is pretty simple: just connect the mic + headphone + SIM card to module and then send the right AT commands to start/end the call (or do whatever you want to do). Not very complicated and you do not need a powerful processor to do those basic operations.