[asterisk-users] Multiple phones when one is unregistered

[D'Arcy J.M. Cain]

I have an extension that looks like this:

exten => 5555551111,1,Verbose(Door buzzer calling)
  same => n,Dial(SIP/user1&SIP/user2&SIP/user3)

The idea is that any of the three users can answer the phone to let
someone in.  The problem is that if, say, user2 unplugs his phone then
the call immediately goes to his voice mail and the other two do not
have the ability to open the door.

Is there any way to direct only to phones in a list that are currently
registered?  I am sure that I can write a rather convoluted extension
to check for registrations and create a dial command but I am hoping
that there is an easier way so that I can create these types of
extensions for other clients easily as well as being able to add and
remove destinations quickly.

Cheers.

-- 
D'Arcy J.M. Cain
System Administrator, Vex.Net
http://www.Vex.Net/ IM:darcy at Vex.Net
VoIP: sip:darcy at Vex.Net