"Linphone Linphone is a mature piece of software, it uses SIP,
for voice and video over IP, it can work as a stand alone
application, and you can call other sip enabled devices just
entering its ips on the dial window of Linphone, or you can
configure it to use an Asterisk PBX.
"There are binary packages for the most common distros, so try
to install it using you package manager.
Ekiga, is the softphone for Gnome Desktop, it supports both SIP and
H.323, also has support for webcams if you kernel recognize
"As with Linphone you may use Ekiga standalone, or with a
softswitch, you can also create an account using the Ekiga
configuration druid, but over all, it is a great soft phone to be
used with Asterisk. Here is a screencast to learn how to configure
it, to work with Asterisk."