When using DNS SRV records and 1st destination no response then it try to 2nd destination. If 2nd destination rejected the call with 486 then kamailio(version: 4.1.1) forward this code as 503. In config we use failure_route as,

if (t_check_status("408")) {                                                                                                                        t_reply("503","Service Unavailable");                                                                                             }

Could anyone please explain this scenario? why this 486 translated to 503?