1. Google Alchemy.
2. Google open source C/C++ SIP SDK or Your own written SIP C libraries
3. Convert them using Alchemy to SWF
4. Flex/Flash use those libraries
Alchemy + C/C++ Source code == Flex Bridge
Most solutions use an RTMP to SIP gateway rather than implementing SIP on the client e.g. http://npramesh.wordpress.com/2008/05/20/open-source-sip-phone-with-red5-and-flex3
It would be much more scalable if SIP was supported directly in AIR but I'm not aware of any projects doing this yet.
Another option to using Alchemy would be to have an AIR application implement the presentation layer over a Java SIP softphone e.g. SIP communicator. This would require integrating flex with Java (http://www.adobe.com/devnet/flex/flex_java.html) (http://code.google.com/p/flerry). I'm not sure how feasible this is for you but I think it would work in theory.