Long time a ago, I used Ekiga a.k.a. GnomeMeeting, but it does not exist anymore.
It was very easy to use and looked good.
Then I moved to Linphone, but the GTK UI does not exist anymore.
I’m now at Twinkle, which is OK, but it uses Qt, which fits less nicely in my GTKish desktop.
I would also be happy about a curses based SIP client, something like
mutt, but for phone calls
Which softphone do you use?