[Kamailio-Users] Question regarding TM module DNS failover with SRV records

According to the 1.5.x TM module documentation, DNS failover is triggered by a 503 reply or a timeout. When using SRV records, it appears a 503 triggers the failure_route instead. Given: host -t srv _sip._udp.voip.example.net _sip._udp.voip.example.net has SRV record 1 0 5060 fast-server.example.net. _sip._udp.voip.example.net has SRV record 2 0 5060 slow-server.example.net. On an invite to voip.example.net, if fast-server does not respond within "fr_timer" seconds, t_relay fires off a serial branch (t_on_branch is triggered) and the invite goes to slow-server. If fast-server responds with a 503, the t_on_failure route is triggered instead, and the invite is not sent on to slow-server. If its not a bug in the module, does anyone have clues of how to coax the next DNS SRV record out of the t_on_failure route? Or what can be done to get t_relay to consume the 503? Thanks!