According to the OpenBTS Project, OpenBTS is an open-source Unix application that uses the Universal Software Radio Peripheral (USRP) to present a GSM air interface ("Um") to standard GSM handset and uses Asterisk software to connect calls leveraging SIP and VoIP.The goal of the project is to reduce the cost of GSM service provision in rural areas and the developing world. They explain, "In plain language, we are working on a new kind of cellular network that can be installed and operated at about 1/10 the cost of current technologies, but that will still be compatible with most of the handsets that are already in the market. This technology can also be used in private network applications (wireless PBX, rapid deployment, etc.) at much lower cost and complexity than conventional cellular."
During the past few Burning Man festivals, OpenBTS has performed a week-long live field test under special temporary WE9XJN authorization to use certain wireless frequencies. At the 2010 Burning man they explained, "We ran a 2-sector, 5-TRX system (3/2 configuration) from a 25 m tower. We ignored RACH bursts with TA>10, limiting our range to 5 km, deliberately excluding nearby towns from the test." They added, "We encountered roughly 40,000 unique IMSIs. Really. We were shocked, too." Also they stated, "We had about 4,000 autoprovisioned users, connected about 7,000 phone calls and processed about 50,000 text messages."
So now you can use Asterisk + OpenBTS to build your private cellular network. Just make sure to turn it off periodically before a bunch of government vans triangulate your illegal cellular network. 
What can't Asterisk do?
