I just discovered a very cool open-source project that turns an ordinary laptop into a cellular tower. It uses Asterisk to route calls.
Very, very cool. Now I have to find out how it works!
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 the Asterisk® software PBX to connect calls. The combination of the ubiquitous GSM air interface with VoIP backhaul could form the basis of a new type of cellular network that could be deployed and operated at substantially lower cost than existing technologies in greenfields in the developing world.
via The OpenBTS Project.