[Serusers] Question about failure route

I am trying to implement a Failure route as follows:

modparam("tm", "fr_inv_timer", 15) modparam("tm", "fr_timer", 10)

if ((uri=~"^sip:1[0-9]{10}@.*")) { route(4); break; };

route[4] { rewritehostport("fisrtgatewayip:5060"); t_on_failure("1"); t_relay(); break; }

failure_route[1] { rewritehostport("secondgatewayip:5060"); t_relay(); break;

}

But I see that Failure route is never taken. After a SIP trace I see that SER always reply with 408 Request Timeout to the UA. Is there something wrong with routing logic or is there a way to avoid 408 message to UA.

Please help!

Thanks