I use sngrep to track view the flow and I'm pretty sure it's accurate enough to tell me that.
Here's relay route:
route[RELAY] {
# enable additional event routes for forwarded requests
# - serial forking, RTP relaying handling, a.s.o.
if (is_method("INVITE|BYE|SUBSCRIBE|UPDATE")) {
if(!t_is_set("branch_route")) t_on_branch("MANAGE_BRANCH");
}
if (is_method("INVITE|SUBSCRIBE|UPDATE")) {
if(!t_is_set("onreply_route")) t_on_reply("MANAGE_REPLY");
}
if (is_method("INVITE")) {
if(!t_is_set("failure_route")) t_on_failure("MANAGE_FAILURE");
}
if (!t_relay()) {
sl_reply_error();
}
exit;
}
and here's reply routes
# Manage outgoing branches
branch_route[MANAGE_BRANCH] {
xdbg("new branch [$T_branch_idx] to $ru\n");
route(NATMANAGE);
}
# Manage incoming replies
onreply_route[MANAGE_REPLY] {
xdbg("incoming reply\n");
if(status=~"[12][0-9][0-9]")
route(NATMANAGE);
}
# Manage failure routing cases
failure_route[MANAGE_FAILURE] {
if (t_check_status("486")) {
exit;
}
route(NATMANAGE);
if (t_is_canceled()) {
exit;
}
}
However when endpoint replies with 486 BUSY I can't see that on FS, Kamailio just sends 408 REQ TERM after some amount of time