route
{
  # ...

  /* call to PSTN */
  if( /* is phone call */ )
  {
    if(!load_gws("1"))
    {
      sl_send_reply("500", "Internal server error, unable to load phone gateways");
      break;
    }
  }
  else /* is fax call */
  {
    if(!load_gws("2"))
    {
      sl_send_reply("500", "Internal server error, unable to load fax gateways");
      break;
    }
  }

  if(!next_gw())
  {
    sl_send_reply("503", "Service not available, no gateways found");
    break;
  }
  
  t_on_failure("2");

  t_relay();
}

failure_route[2]
{
  if(method == "INVITE" && t_check_status("408|500|503"))
  {
    if (!next_gw())
    {
      t_reply("503", "Service not available, no more gateways");
      break;
    }
    t_on_failure("2");
    t_relay();
  }
}