Smartphones aren't expensive these days, but if you'd prefer to build your own, DIYer David Hunt shows off how to use a Raspberry Pi as a phone.
The build here costs $US158, which means it's probably cheaper to buy a phone off the shelf. Still, this one's fully customisable. It uses a touch screen, a Raspberry Pi, a GSM/GPRS module, and a few other parts. You'd obviously need to build a case for it to actually use it, but if you're in the mood to create your own phone (and learn some extra electronics in the process), this is worth investigating.