Yes, it is possible to build a cell phone that can perform the functions of making calls, sending SMS, and even data transfer. For an example of a GSM module that would be the main component of such a system, see here: <a href="http://www.sparkfun.com/products/10138" rel="nofollow">http://www.sparkfun.com/products/10138</a><p>However, from a cost and performance point of view, it doesn't make any sense to do so. The cell phone OEMs have a large amount of engineering and logistics resources to put behind building devices that are aesthetically pleasing, small, and powerful.
You can and a simple Google search will link you to a number of homebrew projects.<p>Hint: include terms like "avr", "Arduino", or "arm" in your searches.<p>The problem with home brew cellular systems is, unless you have access to some pretty advanced chip/board fab equipment, they're usually way too bulky to be practical, and they use too many mAh's (battery life).<p>The stuff on Sparkfun and Adafruit are great for hobbyist projects, but aren't going to be better than a cheap series 40 Nokia that's hackable via a serial interface.
major chip manufacturers probably have reference designs you can get, which include some of their parts, and which should be a good starting point.<p>you will indeed need to have access to advanced PCB fab and assembly subcontractor, but with some digging around you can find those.<p>the biggest problem is that you'll need hardware and software engineers who really know their job and can do the schematics, layout and firmware - doing a cell phone well is not an easy or small project, certainly not if you want to do something at a serious level and go beyond hobbyist type hacking.<p>you'll also have to do research on where to get an OS - developing this yourself is a huge project, probably just as big if not bigger than doing the hardware design. I don't know if it's possible, but perhaps you can get android OS source code and use that as a starting point.<p>you'll have to make each part of the hardware talk to the OS which means writing some driver or glue code, which is a whole other job. I'd try to use standard peripherals/ICs in your design for which android already has drivers.<p>without knowing more about your project, I can't really help more than this - if you want to talk about it feel free to msg me.