http://www.linux-ha.org/wiki/Main_Page
http://www.voip-info.org/wiki/view/Asterisk+High+Availability+Solutions
http://www.cisco.com/en/US/tech/tk652/tk701/technologies_white_paper09186a00800a9818.shtml
http://ohlmeier.com/work/Des_Impl_HA_SIP_Arch.pdf