Stun

http://www.pernau.at/kd/voip/bookmarks-sip-stacks.html comparation of SIP client libraries – not much of a comparation but a list of available SIP stacks with short notes

http://www.freepbx.org/support/documentation/howtos/howto-setup-a-remote-sip-extension notes on how to setup a “remote extention”, i.e. you have asterisk PBX in the office, but you want to place 1 IP Phone at home. “IP phone at home” is the “remote extention”. Notes explain how to work around NAT gotchas.