[Users] how to configure failure route for b2bua or GW

hi all I try to build failure route to another backup b2bua or pstn gw,it seems can work after timeout ,but unfortunate,the openser send a 408 messeage to phone before failure route.,config like that,have any general config to failover or loadbalance for me.Many many thx. route[4] { log(1, "b2bua"); rewritehost("192.168.1.204"); forward("192.168.1.204:80"); t_on_failure("2"); t_relay(); } failure_route[2] { log(1, "b2bua"); rewritehost("192.168.1.226"); forward("192.168.1.226:5060"); t_relay(); }