In this presentation from the just concluded 29C3 conference in Hamburg, Germany, speaker Sylvain Munaut explains how to turn a cellphone into a BTS.
The Calypso baseband and its companion chips are used on the Motorola C123 among other and are now well known for being supported by the Osmocom-BB open source GSM baseband implementation. A couple years ago, it was hacked a little further by using it as a raw bits capture device allowing the interception of GSM traffic very cheaply.
This talk will present some further work on that platform, showing that just because a device wasn’t design for a given task doesn’t mean it can’t do it. More specifically how you can hack this phone to act as a GSM basestation and broadcast your own network.
For additional info on hacking the Motorola C123, see the OsmocomBB C123 page.