Ahoi,
I am currently developing a Flash client for SIP communication. My biggest help on the subject is the following website,
where I have found a great sample program:
http://www.voip-sip-sdk.com/p_97-flash-sip-client-example-voip.html
The sample program is about how to make a Flash softphone for SIP communication. I hope you will like it as much as I do.
Greetings,
Gedeon