Can someone tell me why the pseudo variable $rr (SIP reply reason) is not present in a failure_route?

If a call fails, this pseudo var contains the SIP response code, '404' for a Not Found for example. I would think that it should DEFINITELY be set in a failure route. OTHER pseudo variables are set in a failure route. Why not the SIP response code? This is probably the most important pseudo variable to have available in a failure route!

Douglas.