Asterisk is a PBX system I am working with lately at Vontoo. Vontoo is in the business of empowering organizations to go beyond direct mail as their medium of communication and bring the emotional impact of voice to the table.
Asterisk is a new system for me, so I am just starting to figure it out. Most of what I have figured out is on the SIP client side. I initially tried using Ekiga on Windows. Ekiga however is built on GTK+ and wants to use a version different than the version used by Pidgin. GTK+ does not currently use the side by side assembly functionality of Windows. The result is that multiple GTK+ applications running against different GKT+ versions cannot coexist without problems. Ideally, GTK+ will eventually be deployed in a side by side assembly form, and Pidgin can have a manifest telling what version to use. Ekiga could do the same. The two could then run side by side without problems.
Moving on, I found Gizmo5 which is similar to Skype, but it allows you to also connect to your own PBX such as Asterisk as well. My colleague and I have not successfully set up Asterisk yet, but I was able to use Gizmo5 at least with the Gizmo5 service as well as my Google Voice account. The work will continue tomorrow.