I have not tried Fring, but I found the best quality to be the rtcom client. And yes there is that issue of not hearing the other person sometimes, but there is a lot of threads that have talked about this. I found that if I leave the internet call application open and just minimized (as you would with fring, or gizmo) it works perfectly every time. Another guy say when he can't hear the other person he messes with the volume and suddenly he can hear them. It is a weird qwirk with the application. But the lightweightness and auto log in when I roam to other wifi hotspots is so perfect. The sound quality is excellent for me. Maybe you should check(play with) your advanced settings. Also it could be your internet connection.
Again I have not tried Fring but I am a huge advocate for rtcom having been a huge skype and gizmo user.