rfc 3261 (SIP) libraries for arduino

Anyone know of any Sip Stack library's for the arduino + ethernet shield? I would like to interface the arduino into an asterisk system, but I can find much information. Is this a idea even feasable?

Thanks!

I recall the mention of asterisk and arduino only once on this forum, a google "asterisk site:arduino.cc" gave - http://arduino.cc/forum/index.php?topic=57499.0

might help?