asterisk users - Nov 2005 - Detect registered peers

Hi,

Is there a way to detect (in the dialplan) if a SIP peer is registered 
with the server ?

I am using macros to dial to extension, becuase i dont want to define 
each extension in the dialplan, and, for example, my numbers are 8xx , i 
  want to know if a peer exists/registered before ringing the line, i 
need something like Voicemailexists , but for SIP peers.

any solution ?

Thanks.

Marco.