UPnP and NAT-PCP both support creating port mappings for other hosts/IPs. Could you add a new type of operation that uses UPnP/NAT-PCP to create port mappings for UA so that RTP traffic can flow directly between the UA and the remote endpoint?
OpenWrt users would then be able to install siproxd and miniupnpd to avoid NAT problems without having to forward RTP traffic in the router which may not be able to handle the load well given the relatively weak hardware. This approach is much more scalable, and has little overhead, it would be possible to have concurrent calls without using much system resource.
miniupnpd secure_mode=no allows adding port mapping by other IPs
https://github.com/miniupnp/miniupnp/blob/master/miniupnpd/miniupnpd.conf
Information about NAT-PCP "THIRD_PARTY"
https://tools.ietf.org/html/rfc6887#section-13.1
