Unable to make call using firefox with url "sip:(number)"

I am using Ubuntu 8.10, 32-bit version with Firefox 3.0, i have install Ekiga 3.0.1, i have noticed when i click in sip urls like "sip:<email address hidden>" , ekiga does get open but does not dial the call to address...

i have tried to open the url explicitly in Firefox 2.0 , 3.0 but having same problem...

with other software it is not so.. they do dial the sip address...

is it the bug or unavailability of a feature ?

i m able to make call using command ekiga -c <email address hidden> i want to do it using browser...