SIP registration saga continued

I am currently using the Linksys PAP2 adapter because it has 2 SIP accounts allowing me to connect to either callcentric or gizmoproject. I have only one phone connected but switching the phone cord is easier than changing the configuration everytime. Any way, I noticed yesterday that sipsak on the PC was able to query proxy01.sipphone.com without problems. But Linksys was still complaining that it couldn't contact the login server. Rebooting the Linksys didn't help. So I tried tshark on the PC and ran sipsak. In addition to the response to sipsak request, I noticed something strange. I was getting "401 SIP Status Unauthorized" packets coming to my PC. The content looked like some kind of Challenge for authorization which I confirmed by googling.

So it turned out that the BSNL Huawei modem had a messed up NAT. Response to the outgoing registration packets from the Linksys were coming to the PC! So I rebooted the modem but that didn't help either. I just retyped my password in the Linksys and that did the trick. I am reasonably confident that it was NAT translation issue; so BSNL is not at fault on this occasion. At least not since yesterday.