fixed one-way RTP proxy in case of failure route

etc/ser-oob.cfg

@@ -258,6 +258,7 @@ flags
   FLAG_SERWEB_RSVD2   : 10, # bit reserved for use with serweb
   FLAG_SESSIONTIMER   : 11, # indicates that the UAC supports Session Timer
   FLAG_RR_DONE        : 12; # the request got already one RR header
+  FLAG_RTP_PROXY      : 13; # the RTP proxy is turned on
 
 avpflags
   dialog_cookie;        # handled by rr module
@@ -629,17 +630,17 @@ route[RTPPROXY]
 	}
 
 	# if the message terminates a dialog turn RTP proxy off
-	if ((method=="BYE" || method=="CANCEL") ||
-		isflagset(FLAG_FAILUREROUTE)) {
+	if (method=="BYE" || method=="CANCEL") {
 		unforce_rtp_proxy();
 		append_hf("P-RTP-Proxy: UNFORCED\r\n");
 		break;
 	}
 
 	# turn the RTP proxy on for INVITEs and UPDATEs
-	if ((method=="INVITE" || method == "UPDATE") && @msg.body != "") {
+	if (((method=="INVITE" || method == "UPDATE") && @msg.body != "")  && !isflagset(FLAG_RTP_PROXY)) {
 		force_rtp_proxy('r');
 		append_hf("P-RTP-Proxy: YES\r\n");
+		setflag(FLAG_RTP_PROXY);
 	}
 }
 
@@ -1190,7 +1191,7 @@ onreply_route[REPLY_ROUTE]
 
 	# if a NAT is involved and this is the final positive reply
 	# which contains a body, start to use the RTP proxy
-	if (isflagset(FLAG_NAT) &&
+	if (isflagset(FLAG_RTP_PROXY) &&
 		status=~"(18[03])|(2[0-9][0-9])" &&
 		@msg.body != "") {
 		force_rtp_proxy('r');