An enterprising tinkerer named David Hunt built a “smartphone” from a Raspberry Pi and off-the-shelf parts. Appropriately, he named the contraption the PiPhone, and it actually functions.
Hunt used an Adafruit touchscreen as a display and popped in a SIM900 GSM/GPRS module to enable phone calls, send texts, and use data. There’s a 2500mAh LiPo battery and a DC-DC boost converter as well as a few cables, connectors, and switches, too.
Yes, the PiPhone is hideous (someone please 3D print a case for this thing), and yes, you can acquire a much better phone from any provider for less than the $158 Hunt spent on parts, but there’s something delightful about seeing someone figure out how to make a functioning cell phone from a pile of parts.