Rated 4 out of 5 stars

I have resolved the problem about the messagge
"to use this feature go to SJLabs and install SJPhone".
SJPhone is installed and works fine.
So I used RegMon to search something of wrong...
I have found that thunderbird (SJCall plug-in)
was reading for HKLM\SOFTWARE\SJLabs\SJvoip Project\SJphone\Path, but this is not available.
You resolve the problem adding the following reg file (modify the path if it's necessary):
[HKEY_LOCAL_MACHINE\SOFTWARE\SJLabs\SJvoip Project\SJphone]
"Path"="C:\\Program Files\\SJphone 1.65\\"

Restart Thunderbird and... call!