It must run on Windows XP/Vista/7 and OSX/Linux would be an added bonus. Java and flash are available to work with.
Seems to me java has a SIP API so it might be the best route. I'm not opposed to exploring the use of Adobe Flash but I don't believe it supports it natively and might be too complicated to get setup. Please leave a comment indicating what you're thinking this should be built in.
Preference will be given to those who can demonstrate they have a solid understanding of my needs and are able to complete this project without issue. I don't expect you to support the software once it is built but if you're available for future consulting please advise. Full source code (where available) will be required upon successful completion of this project (sloppy coders beware.)