if($siproxd_conf['port'] != "") {
		fwrite($fout, "sip_listen_port = " . $siproxd_conf['port'] . "\n");
	} else {
		fwrite($fout, "sip_listen_port = 5060\n");
	}

ADD:

        if($siproxd_conf['host_outbound'] != "") {
		fwrite($fout, "host_outbound = " . $siproxd_conf['host_outbound'] . "\n");
	}

/usr/local/pkg/siproxd.xml -
AFTER the block:

		 <field><fielddescr>Listening port</fielddescr>
			<fieldname>port</fieldname>
			<description>Enter the port on which to listen for SIP traffic (default 5060). Do not change this unless you know what you're doing.</description>
			<type>input</type></field> 

ADD:

		 <field><fielddescr>External WAN Static IP</fielddescr>
			<fieldname>host_outbound</fieldname>
			<description>Enter IP of the outbound interface.</description>
			<type>input</type></field> 

Picture:

Would suggest this patch are merged into the main siproxd package since this can be useful for "behind NAT" setups where you know the external IP. Then its a waste of Resources to use a STUN server.