Spoofing GSM Cell Base Stations

There was the fellow demonstrating a spoofed 2G GSM tower at DefCon 18:

http://www.forbes.com/sites/firewall/2010/07/31/despite-fcc-scare-tactics-researcher-demos-att-eavesdropping/

And the YouTube video was pretty cool to watch, for technical depth.

https://www.youtube.com/watch?v=rXVHPNhsOzo

Part of his talk described how he could (up a point) block 3G, to
cause phones to fail over to 2G, where he could get them.