Ekiga Mumble opensoftphone

I just checked and Twinkle is not in Debian Testing.

I would personally vote or GoogleTalk/GoogleVoice.

Another vote for Skype, who'd have guessed, a Microsoft application getting a vote on LQ‽ Just heard it's beginning to get a little chilly in hell.
But in all seriousness, it's not Open in any way, but as everyone uses it, and the fact it actually still is supported on GNU/Linux based systems, even with the new owners, is pretty awesome.

I'm surprised by all the love for Skype I see here. Yes, it's useful, but that's just market share: if I want to talk to everyone else, I have to have it and (grr) make it work. But I'd hardly call it worthy of an award. Development on Linux is dormant. Bugs on Linux go unfixed for, literally, years. It only installs and works reasonably well on a small subset of distros. Not only the client is proprietary, the protocol is secret too, so you can't use an open source alternative to communicate with Skype users. And need I mention the new owners?

Skype fills a current vacuum. But like all vacuums, it sucks.

And again no Asterisk here, however "Yate client" was so popular choice!