Generally speaking, Gizmo works much better with headphones than with the built-in speaker. I found that I could get landline quality with gizmo using the built-in mic and standards hearbud headphones (from a mp3 player) AS LONG AS I AM CALLING ANOTHER INTERNET PHONE.

But when I call a landline, quality is quite bad. I usually need to call phones in Europe (France or Germany), so I suppose that the problems come from poor support of those countries by sipphone.