I've juste downloaded Jitsi ... and I am quite pleased with it.
In particular, the fact that I can use Telify to call phone numbers from Thunderbird (contacts) and Firefox.
However, there is one problem: when I call OVH (number 1 french web hosting company) hotline using jitsi and SIP, it never gets through. When I call using my regular land line it works. When I use Zoiper, it works too. Note that using Linphone it does not work either.
The phone number to call is: +33972101007