Nathelper/rtpproxy has been changing rapidly, see recent mailing list messages and check out this example.
jht2 12/25/03

  --------------------------------------------------
  #
  # $Id: ser.cfg,v 1.21 2003/06/04 13:47:36 jiri Exp $
  #
  # simple quick-start config script
  #

   # ----------- global configuration parameters ------------------------

  debug=3         # debug level (cmd line: -dddddddddd)
  fork=yes
  log_stderror=no	# (cmd line: -E)

  /* Uncomment these lines to enter debugging mode 
  debug=7
  fork=no
  log_stderror=yes
  */

  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"

  alias=mydomain.dyndns.org

  # ------------------ module loading ----------------------------------

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


  # -- rr params --
  # add value to ;lr param to make some broken UAs happy
  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 (len_gt( max_len )) {
		sl_send_reply("513", "Message too big");
		break;
	};

	# we record-route all messages -- to make sure that
	# subsequent messages will go through our proxy; that's
	# particularly good if upstream and downstream entities
	# use different transport protocol
	record_route();	
	# loose-route processing
	if (loose_route()) {
		t_relay();
		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;
		};

		# native SIP destinations are handled using our USRLOC DB
		if (!lookup("location")) {
			sl_send_reply("404", "Not Found");
			break;
		};
	};

       #inserted by klaus
       if (method=="INVITE") {
                record_route();
                force_rtp_proxy();
                /* set up reply processing */
                t_on_reply("1");
        };


	# forward to current uri now; use stateful forwarding; that
	# works reliably even if we forward from TCP to UDP
	if (!t_relay()) {
		sl_reply_error();
	};

  }

  #inserted by klaus
  # all incoming replies for t_onrepli-ed transactions enter here
  onreply_route[1] {
       if (status=~"[12][0-9][0-9]")
                force_rtp_proxy();
  }

See also