Informative Links:
ZRTP or Zfone stuff, for secure key exchange over RTP:
en.wikipedia.org/wiki/ZRTP
zfoneproject.com/prod_asterisk.html
www.gnutelephony.org/index.php/GNU_ZRTP
www.voip-info.org/wiki/view/Asterisk+en...
bugs.digium.com/view.php?id=10024
www.gnutelephony.org/index.php/ZRTP_FAQ...
www.mail-archive.com/asterisk-users@lists.digium.com/msg206873.html

SIP stuff:
www.freeswitch.org/
www.freeswitch.org/node/117
wiki.freeswitch.org/wiki/Quick_Start
wiki.freeswitch.org/wiki/What_FreeSwitc...
wiki.freeswitch.org/wiki/Rosetta_stone
wiki.freeswitch.org/wiki/Secure_RTP