I’m a real VoIP user. I’m heavily relying on Skype as my main VoIP tool – as it just works and integrates encryption – but I’m ready to switch to alternatives .. no support for monopolies here 🙂 One real big problem is to find software working from behind NATs and firewalls. It took me some time, but now I’m quite satisfied: I’m currently online using multiple SIP providers. All with different STUN proxy servers but integrated in one tool: Twinkle. Twinkle allows users to register at different SIP service provider (e.g. sipgate.at, sipgate.de, ekiga.net, gizmo, freeworlddialup, etc.) with different STUN proxy servers. This is unfortunaltely not possible with the Ekiga softphone, which only allows one STUN proxy server for all.

Here is a summary of my experience with selected softphones:

• Ekiga (Linux, Windows) – Has replaced the gnome-meeting project. Works fine, but allows only one global STUN proxy server. That’s a problem if you want to register to different SIP providers from behind a firewall. The feature has been promised in the next major release, which might take some more time.
• Gizmoproject (Linux, Mac, Windows) – This is lame. Needs lot of memory and wastes CPU cycles. Really slow startup. Does not allow to register to other SIP providers.
• Twinkle (Linux only) – Works fine. Has all features I could think of using. The interface is very basic and could be somewhat ‘cooler’
• FireFly (Windows only) – Didn’t get it to work with other SIP service providers. Freshtel might have stripped this feature in later versions (which I have tried). I wouldn’t recommend it.
• X-Lite 3.0 FREE Softphone (Linux only 2.0, Mac, Windows) – Without firewall it works like a charm but I didn’t get it to work from behind the firewall. It does not support multiple registrations in the free version.
• OpenWengo (Linux, Mac, Windows) – WengoPhone includes some IM functionality (ICQ, AIM, Jabber, Google Talk and MSN).
  Updated: As for OpenWengo 2.0 (can be installed using apt-get): You need to create a Wengo account first as without one they won’t let you configure the whole thing. Although the homepage states that multiple registrations with other SIP providers are supported I did not manage it.
  As for OpenWengo 2.1.1: It works fine with Sipgate accounts (have tried those), but you cannot register at multiple SIP providers.

My recommendation is: Use Skype as long as you can and then switch to Twinkle. Both ‘work’ but Skype has a better interface and does not need any (major) configuration. However Twinkle is kind of ‘swiss army knife’ SIP client. Eventually Ekiga will bring support for giving a proxy for each SIP provider. Then I might switch to Ekiga.