Narendra Sisodiya, Simply using common protocol(SIP) does not guarantee that all these different SIP clients will be inter-operable. VOIP is a complex subject. there are various other things in the puzzle the most important being audio codec. skype uses "SILK: Super Wideband Audio Codec"[1] which I would argue is better than any other open source codec out there.
If you want learn more on VOIP, watch FLOSS Weekly Ep 162. Where the host Randal Schwartz(Perl Guru) talks to Emil Ivov(Leader of Jitsi project) about Jitsi. Jitsi open source communicator for instant messaging, audio. There is lot of information on the how VOIP functions, what the challenges etc. Its really fascinating topic [2] Regards, Gaurish Sharma [1] http://developer.skype.com/silk [2] http://twit.tv/floss162 -- LUG@IITD - http://lug-iitd.org/Footer
