Overlord, you are completely wrong.
In fact, not only is this possible, you guys just had MAJOR GOVERNMENT SECURITY FIRM hacked while they were on a conference call.
There, infact, is an operating system that is BUILT around intercepting internet traffic and VOIP traffic.
Skype sends data, therefore it is VERY possible to intercept, insert, and join that data stream.
Anyone who has ever sent a packet of data using any method of programming in their life is aware of this, and, despite encrypted protocols, data can STILL be taken and read.
Don't flaunt development experience and then stand by the idea that nothing is breakable, and all things are flawless. Its nieve.