I have just released the next beta version of the Red5 Plugin with an interesting feature. It is a SIP softphone implemented in Flex and Red5. It is completely open source and should work on both Windows and Linux. I however done all my development and testing on Windows. The voice quality needs more work, but is usable. Codecs are Nellymoser Asao from Flash to SIP and ADPCM from SIP to Flash. It should work with mot SIP proxies, but I have only tested with Asterisk. I have also only tested 2 simultaneous users, but there is no limit and will depend on how many users and media streams Red5 can cope with before it dies.
My next task will be to integrate into Sparkweb and make it work with the SIP plugin and enable SIP calls between Spark and Sparkweb users using the same user profile. For window users, I will also add AsteriskWin32 to the plugin and provide a complete SIP solution for Openfire.
You can download the Red5 Plugin version 0.0.15 from here