http://wiki.kolmisoft.com/index.php/Asterisk_Background_performance_test performance tests for asterisk on 4 Intel Cores @ 2GHz, 2 GB RAM, over sip, recorded media playback – results: saturates 100mb network at ~1000 simultaneous calls, one way GSM playback.
http://laforge.gnumonks.org/weblog/2010/04/14/#20100414-anatomy_of_gsm_cellphone An excellent [attempt of ] description of the GSM cell phones hardware, design, architecture and implementation
http://openbts.sourceforge.net/background.html basically how to build your own gsm cell or private cellullar network on a budget. Very interesting project, requires radio engeening knowlegde and familiarity with telecom and gsm stuff. Also see http://openbts.sourceforge.net/ for links about field tests at Burning Man 2008