Take a look at the specifications of Neo FreeRunner (schematics available) to see what is involved in a cellphone. Basically you need:
- CPU and its associated circuitry (to drive a display, receive touchscreen events etc...)
- RF and baseband circuitry; design circuit boards, antenna etc...
- other hardware (Bluetooth, Wi-Fi, GPS etc...)
- writing software to make it all go together (even if you use Android you will probably need to write drivers).
Some of those devices might be covered by non-disclosure agreements (NDAs) and not be easily available for hobbyists.
If you can do with something very simple (only voice and nothing else), check the DIY Cellphone, which also is open hardware.
The initial prototype combines a custom electronic circuit board with a laser-cut plywood and veneer enclosure. The phone accepts a standard SIM card and works with any GSM provider. Cellular connectivity is provided by the SM5100B GSM Module, available from SparkFun Electronics. The display is a color 1.8″, 160×128 pixel, TFT screen on a breakout board from Adafruit Industries. Flexures in the veneer allow pressing of the buttons beneath. Currently, the software supports voice calls, although SMS and other functionality could be added with the same hardware. The prototype contains about $150 in parts.