asterisk users - Feb 2006 - can't hear 'service messages' when iax is in the middle

If I call a cellular phone while it's off, I can't hear the voice saying
"called number is unreachable", but only if I'm passing trough a
iax
channel.

SIP client ---> Asterisk ---> SIP gateway, works
SIP client ---> Asterisk client ---> Asterisk server ---> SIP gateway, 
doesn't work

(I can't put an explicit Answer in the extension for billing purposes)