I am not sure I completely understand your recommendations.
If you are trying to make VOIP work, then it already works w/o any settings.
Or are you showing me how to QoS the SIP and RTP packets? Just want to be certain of what I am doing.
Your question seemed to be about how to do port mapping for the RTP “connections” associated with SIP initiated VoIP calls and the issue of having more than one SIP end point on your LAN. Basically the conventional advise to map a range of ports for RTP does not work if you have more than one end point on your NATed LAN.
The “modprobe nf_nat_sip” solves that issue. If you have the kernel module loaded then it will detect SIP negotiations and handle the RTP port set up and mapping for you. You don’t need to block out a range of ports and map them to one end point or another.
If I mis-interpreted your question then please excuse my error.