[Serusers] [ser-0.8.14] "100 Trying" and "408 Request Timeout"

[Star Bravo]

Hi all,

(I apologize in case my post is a redundant one)

I am running ser-0.8.14 on a Public IP and trying to make an IP-to-IP
call using Xten X-Lite softphones.

[Call Scenario]
One softphone is running on a Public IP while the other one is behind
NAT running on a Private IP.

[Problem]
When caller softphone sends INVITE packet to SER, SER responds with
"100 Trying" and finally after few seconds SER responds with "408
Request Timeout".
The call does not connect, even the called party softphone (behind
NAT) does not receive any SIP messages from SER.

[Question]
Why SER is not able to relay SIP messages to the called party softphones?

[More Information]
Call works fine if I initiate the call from the softphone that is
behind NAT (using a Private IP)

I am copying my ser.cfg file below.

# ----------- global configuration parameters ------------------------
check_via=no	# (cmd. line: -v)
dns=no           # (cmd. line: -r)
rev_dns=no      # (cmd. line: -R)
fifo="/tmp/ser_fifo"

# ------------------ module loading ----------------------------------
loadmodule "/usr/local/lib/ser/modules/sl.so"
loadmodule "/usr/local/lib/ser/modules/tm.so"
loadmodule "/usr/local/lib/ser/modules/rr.so"
loadmodule "/usr/local/lib/ser/modules/maxfwd.so"
loadmodule "/usr/local/lib/ser/modules/usrloc.so"
loadmodule "/usr/local/lib/ser/modules/registrar.so"
# ----------------- setting module-specific parameters ---------------

# -- usrloc params --
modparam("usrloc", "db_mode",   0)

modparam("rr", "enable_full_lr", 1)

# -------------------------  request routing logic -------------------
# main routing logic
route{

	# initial sanity checks -- messages with
	# max_forwards==0, or excessively long requests
	if (!mf_process_maxfwd_header("10")) {
		sl_send_reply("483","Too Many Hops");
		break;
	};
	if ( msg:len > max_len ) {
		sl_send_reply("513", "Message too big");
		break;
	};

	# loose-route processing
	if (loose_route()) {
		t_relay();
		break;
	};

	if (uri==myself) {

		if (method=="REGISTER") {

			save("location");
			break;
		};

		if(method=="INVITE") {
			record_route();
		};

		if (!lookup("location")) {
			sl_send_reply("404", "Not Found");
			break;
		};
	};
	if (!t_relay()) {
		sl_reply_error();
	};
}

Looking forward to your response and feedback.

Thks & Rgds,
- Bravo