Create a handler for the SIP protocol that uses jitsi
Run in terminal:
Code:
sudo gedit /usr/share/applications/defaults.list
Add the following line at the end:
Code:
x-scheme-handler/sip=jitsi.desktop
Fix jitsi.desktop to accept parameters (telephone url)
Run in terminal:
Code:
sudo gedit /usr/share/applications/jitsi.desktop
Change this line:
Code:
Exec=jitsi
To this:
Code:
Exec=jitsi %u
Associate the SIP protocol
Run in terminal:
Code:
gconftool-2 --set --type=string /desktop/gnome/url-handlers/sip/command '/usr/bin/jitsi sip:"%s"@sip.voip_provider_url.com'
where sip.voip_provider_url.com put the appropriate url for your SIP provider.

Then in terminal again:
Code:
gconftool-2 --set --type=bool /desktop/gnome/url-handlers/sip/enabled true
gconftool-2 --set --type=bool /desktop/gnome/url-handlers/sip/need-terminal false
In Thunderbird/Firefox open the Telify preferences (Tools/Telify/Preferences) and set "Used Protocol" to sip:

Done!