I installed "Ser-0.8.14" + "nathelper" + "rtpproxy" in RedHat 9.0, and add "./rtpproxy" in startup script, when i tried both sipsoftphone in the internet behind NAT that can connect but "no voice" and auto hang-up, but when both sipsoftphone in local network that no problem !

/* Uncomment these lines to enter debugging mode
fork=no
*/
#log_stderror=yes
#debug=4
check_via=no # (cmd. line: -v)
dns=no # (cmd. line: -r)
rev_dns=no # (cmd. line: -R)
#port=5060
#children=4
fifo="/tmp/ser_fifo"

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"
loadmodule "/usr/local/lib/ser/modules/textops.so"
loadmodule "/usr/local/lib/ser/modules/nathelper.so"

# Uncomment this if you want digest authentication
# mysql.so must be loaded !
#loadmodule "/usr/local/lib/ser/modules/dbtext.so"
#loadmodule "/usr/local/lib/ser/modules/auth.so"
#loadmodule "/usr/local/lib/ser/modules/auth_db.so"
#loadmodule "/usr/local/lib/ser/modules/mysql.so"

modparam("usrloc", "db_mode", 0)
#modparam("auth_db", "db_url", "db:/var/dbtext")
#modparam("auth_db", "user_column", "user")
#modparam("auth_db", "domain_column", "domain")
#modparam("auth_db", "password_column", "password")
#modparam("auth_db", "calculate_ha1", 1)
#modparam("auth_db", "password_column_2", "ha1_2")
modparam("registrar", "nat_flag", 6)
modparam("nathelper", "natping_interval", 30) # Ping interval 30 s
modparam("nathelper", "ping_nated_only", 1) # Ping only clients behind NAT
modparam("nathelper", "rtpproxy_sock","/var/run/rtpproxy.sock")

# -- auth params --
# Uncomment if you are using auth module
#
#modparam("auth_db", "calculate_ha1", yes)
#
# If you set "calculate_ha1" parameter to yes (which true in this config),
# uncomment also the following parameter)
#
#modparam("auth_db", "password_column", "password")

# main routing logic
#define NAT_UAC_TEST_C_1918     0x01
/*
* test for occurences of RFC1918 addresses in Contact
* header field
*/
#define NAT_UAC_TEST_RCVD       0x02
/*
* test if source address of signaling is different from
* address advertised in Via
*/
#define NAT_UAC_TEST_V_1918     0x04
/*
* test for occurences of RFC1918 addresses in SDP body
*/
#define NAT_UAC_TEST_S_1918     0x08
/*
* test for occurences of RFC1918 addresses top Via
*/
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;
};
if (nat_uac_test("3")) {
       append_hf("Alex-hint: NAThelper\r\n");
fix_nated_contact(); # Rewrite contact with source IP of signalling
        if (method == "REGISTER" || ! search("^Record-Route:")) {
#if (www_authorize("iptel.org", "subscriber")) {
#       www_challenge("iptel.org", "1");
#        };
       if (method == "INVITE") {
  append_hf("Alex-hint: SDP rewritten\r\n");
  fix_nated_sdp("3"); # Add direction=active to SDP
       };
log("LOG: Someone trying to register from private IP, rewriting\n");
       force_rport(); # Add rport parameter to topmost Via
  setflag(6);    # Mark as NATed
     };
};

if (!method=="REGISTER") record_route();

# subsequent messages withing a dialog should take the
# path determined by record-routing
if (loose_route()) {
  # mark routing logic in request
  append_hf("P-hint: rr-enforced\r\n");
  route(1);
  break;
};

# if the request is for other domain use UsrLoc
# (in case, it does not work, use the following command
# with proper names and addresses in it)
if (uri==myself) {
  if (method=="REGISTER") {
   save("location");
   break;
  };
  lookup("aliases");
  if (!uri==myself) {
   append_hf("P-hint: outbound alias\r\n");
   route(1);
   break;
  };
  # native SIP destinations are handled using our USRLOC DB
  if (!lookup("location")) {
   sl_send_reply("404", "Not Found");
   break;
  };
};
append_hf("P-hint: usrloc applied\r\n");
route(1);
}

route[1]
{
        # !! Nathelper
        if (isflagset(6)) {
            force_rtp_proxy();
     append_hf("NAT: ...\r\n");
        };
        # NAT processing of replies; apply to all transactions (for example,
        # re-INVITEs from public to private UA are hard to identify as
        # NATed at the moment of request processing); look at replies
t_on_reply("1");

        # send it out now; use stateful forwarding as it works reliably
        # even for UDP2TCP
        if (!t_relay()) {
                sl_reply_error();
        };
}
# !! Nathelper
onreply_route[1] {
# NATed transaction ?
append_hf("NAT: tes\r\n");
if (isflagset(6) && status == "(200)") {
fix_nated_contact();
fix_nated_sdp("3");
force_rtp_proxy();
append_hf("NAT: test\r\n");
# otherwise, is it a transaction behind a NAT and we did not
# know at time of request processing ? (RFC1918 contacts)
} else if (nat_uac_test("1")) {
fix_nated_contact();
fix_nated_sdp("3");
force_rtp_proxy();
append_hf("NAT: test1\r\n");
};
}