Replacing Skype, that is, to offer a public service anyone can use with public protocols and published standards that includes free software clients, and is fully cross-platform, is in one sense rather easy to do today. Someone recently suggested gnome meeting with ekiga.net for this purpose. I might suggest considering jitsi (http://www.jitsi.org), which is cross-platform, and includes the GNU ZRTP4J stack, either along with iptel.org (http://www.iptel.org), or even better, a similar kind of service powered by GNU SIP Witch, if one were looking for something that does simply this, and just call it a day.
However, simply replacing one “enclosed” network with another, even if one that is using free software and published protocols, simply makes a fundamental barrier to people communicating with each other become even more obvious. What happens if some of the people you communicate with choose ekiga.net, some choose iptel.org, and others choose something else? Does one go back to multiple accounts in multiple places? Does one require network effects to in effect “appoint” a new widely used communication monopoly to replace the old one with critical mass just for people to be able to reliably find and communicate with each other, until that single point of commonly chosen connectivity fails for some reason (whether commercial, political, technical, or otherwise) also?
This is where GNU Free Call most clearly differs from most others who are looking to replace Skype. As GFC is already designed for use with any SIP capable client, we have no interest in re-inventing protocols or even how VOIP clients work. This is not the problem we are looking to solve. We are instead interested in, not simply having people join or connect through yet another specific service provider to mediate their communications (whether iptel.org, ekiga.net, etc), but rather in enabling anyone to discover and communicate with each other directly without the need for a mediating service at all. It is how users are empowered to discover each other which is most important in GFC’s design. This is best illustrated by the GFC client, which is in reality contact focused rather than communication driven. This I think becomes more clear from the GFC GUI design (and experimental client), as illustrated here.
What I think becomes immediately clear is that we are not looking to replace Skype, that is, as a service or facility, but rather instead we are seeking to replace the entire paradigm that originally lead to Skype, and that is still fundamental in many of the other alternatives presently being discussed.