Hi Marcelo,
All the options mentioned on this thread require central media servers. this kind of setup is generally poorer quality and less reliable than peer-to-peer options like Skype. The only caveat being that all users must have a peer-to-peer VoIP client installed on their computer. I've been using VoIP for years - they're usually free and you can try them out without any initial outlay. When you have a good idea of what you need, you can decide if VoIP clients are appropriate for your particular circumstances or whether you need to contract a media service provider.
Just my €0.02!