Hi. I can do it for a fixed fee of 300 $US in 10 days.
I will code it in Python/Qt, which means that the same code base could run on Windows (XP, Vista, 7 and 8), Mac and Linux. Windows version would be compiled into single exe file and wrapped in easy to use self-extracting installer.
Please give me more details regarding the protocols you want me to support (SIP...). Also, If you have a GUI mockup, that would be great.
Thanks,
Dejan a.k.a. Yagosh