asterisk users - Jun 2008 - SIP Phone, multiple line, but one call at a time ?

Hi !

I got some users how have SIP hard phones (aastra for one and nortel trough 
a Citel Portico TVA for the other), who doesn't want the phone to ring when 
they are on the phone. I still need for the "line" to be displayed at
least
2 times on the phone for them to make transfer and conferences, so I just 
cannot make the sip acount only availaible on one button...

Is there any variable that could tell me the "status" (i.e. InUse) of
the
phone that I could use in my dialplan ?

Any other ideas ?

Thanks,

Nicolas