Incoming calls go to failover number

FWIW, the screenshot shows both NAT Mapping and NAT keep-alive disabled.
Changing those to Yes (enabled) was what fixed my problem...

So it turned out to be a firewall (port blocking) issue after all........your router's firewall was closing up the UDP ports after a period of inactivity. By enabling the NAT Keep Alive, the ATA keeps the UDP ports open in your router's firewall by periodically sending data packets to the SIP server.

tritch,

I am sure you understand the difference between firewall and NAT functionality (I saw your posts, and I am saying this without any sarcasm). Obviously, there is a firewall on the router, and obviously it blocks all unsolicited incoming traffic. That functionality is expected and doesn't make it a firewall (port blocking) issue.

If I needed to open any ports instead or in addition to make NAT changes in ATA - then it would be a firewall issue. Heck, if I had to make any changes on the router...

But if it makes you feel better - yes, it turned out to be a firewall (port blocking) issue after all and I was an a$$ to not listen to your advice from the very beginning. Should have started tinkering with the router last week - I'd be in much better position now!