I don't recommend the Zfone standalone Linux beta for VoIP in its current form for desktop users, given that VoIP clients with Zfone built in are becoming available. Ekiga version 3.00 or later has the zrtp libraries built in, and should be available "Real Soon Now" via the Debian and Ubuntu repositories. Unfortunately, version 2.12, which doesn't have these libraries built in, is currently distributed with Debian and Ubuntu. It can't be removed without removing the Gnome desktop, so you'll have to wait until version 3.00 or later is available for upgrade through the repositories.
Unless you have someone or a group for which secure VoIP is absolutely critical, I recommend getting a VoIP program that has the Zfone zrtp libraries built in or waiting until the Zfone release version comes out, which will presumably either be binaries ready for prime time—or, better yet, available via unofficial repositories on the Zfone project site. However, if you need secure VoIP now, these instructions should work for you.
For more information, consult the Zfone FAQ.