[Asterisk-Users] blocked caller id

[Walt Reed]

On Fri, May 21, 2004 at 11:24:04AM +0200, Philipp von Klitzing said:
> Hi!
> 
> > exten => s,1,gotoif,"$[${CALLERIDNAME} = \0]?2:3";
> 
> Do this:
> GotoIf($[foo${CALLERIDNAME} = foo]?2:3)

Not knowing how the low-level caller ID works, how do you differentiate
between "anonymous/private" where the caller has blocking on, and
"unknown/unavailable" where caller ID info is not set (corporate PRI's,
etc.)?