How about making your own smarphone? Making a cell phone or smart phone is not out of reach for clever hardware hacker. If you are planning to make a simple cell phone, ArduinoPhone project shows how you can build one. Or you can check the platforms used to build the cheapest cell phones.
Are you looking for something smarter? If you have a Raspberry Pi and an Adafruit TFT, you’re halfway to making your own version of [Dave]‘s PiPhone. PiPhone is proof-of-concept cellular sandwich is made by adding a Sim900 GSM/GPRS module, which communicates via UART, to the Pi/TFT hardware while using a piece of foam core board in the middle to prevent shorts. The device is powered with LiPo battery and using a DC-to-DC converter to set up the 3.7V to 5V. The build here costs $158, which means it’s probably cheaper to buy a phone off the shelf.
If you plan to go with Raspberry Pi phone route, check also PiUi project.