Tom, you’re right - the other thread symptomatically looks similar but could be different underneath. The thread doesn’t document Asterisk versions in play, so can’t be sure.
Thanks for explaining the interpretation of the two fields. I was under a wrong impression that probably Addr->IP is derived from Reg. Contact in someway, but probably per your explanation, they are two independent fields populated independently. (An aside, can you please point me to any document that explains all these fields and their source of information?). Is there any line of investigation you can suggest to carried out on the Router? What typically should be looked at? I could not find a explicit NAT/SIP related settings on it. I could enable logging on it and see if it throws anything.
All boxes are in different locations; they deploy on-premise and sit in client’s existing networks, over wire or wirelessly. Our box is also an integrated wireless router in itself, in addition to UC/PBX. However, asterisk/freepbx is configured under prescribed norms. As I stated earlier, on networking front, there are four local networks, NAT is set to route for SIP peers as well as trunks (in Advanced as well as SIP and Extensions settings for all users).
Sorry if this information is insufficient to draw some ideas or suggest an investigative path. Please suggest whatever best you can and feel free to ask for any specific data/log that can help assess. Thanks.