asterisk users - Jun 2006 - Trunk failover

Hi,
I'm doing some experiments with SIP vs. IAX2 trunks as an alternative to
PSTN and I noticed that, if voip link is down, failover to PSTN is almost
immediate with the SIP trunk and VEEERY slow with the IAX trunk.
Is there a specific reason? Some timeout to set in iax.conf?

Thanks
-- 
Domenico Viggiani