I have a SIP telephone, but for some strange reason it is not work correctly when it is behind the ROS based NAT. Signalling seems to work as I can phone out or receive calls, but as soon as you try to answer the call there is the silence. Sometimes it does get the voice data going as well, but it is usually one way only (one does hear the other talking, but talking back is no success) and as soon as you try to repeat it, you'll get silence both ways. At the same time if to replace the ROS device with some random Linksys, it is working fine.
For the ROS NAT I use nothing special, the plain "srcnat masquerade" is in use. What is the needed magic with the SIP protocol to get it over the ROS NAT?