Most of them were either:
- not working, properly or at all
- had a horrible interface
Ekiga is in the first category; I couldn't make more than one phone call per application run.
Linphone is in both categories.
I tried others that I don't remember.
At the end I found two working ones:
Twinkle has a clunky and outdated interface, with at least one serious bug (segfault when trying to delete one contact). It's simple to configure, though.
Qutecom has a modern interface, and it works reasonably well.
I again found a bug, but minor - if I try to close the application without hanging up a call, it hangs.
At the end, I chose Qutecom.
For people using Qutecom and Voipstunt, the only parameter to configure (aside the obvious login/password) is "sip.voipstunt.com" as SIP Domain/Realm.
Voipbuster, from the same phone company, uses most probably "sip.voipbuster.com".
The package is in the main Ubuntu repository, so users can install it using the GUI application or command line.