What is so complicated about it ?

The voice is convered to data and xmted across the net...

The receiver changes data to voice and listens to it....

so all you have to do is convert, and xmt to the desired computer using it's IP address...

If you want to use names/Virtual Tel No.

Then, you will have to set up a server keeps a log of logged in user's IP....

For more details, call up a some VOIP service provider and stalk them...

Hope this helps....