I have a WNDR4500 (or N900) with Obi202 and GVoice
Same issues.
I set the port that the Obi is on to be in the DMZ
http://www.routerlogin.net/WAN_wan.htmDefault DMZ Server = IP Address of your Obi device (.103 in my case)
This seemed to work for me.
What i'm concerned about is that the N900 is set up to serve DHCP addresses, so I don't know if the Obi will always be on 103.
The frustrating thing is that in most other routers I've used I can select the physical hardware ethernet port on the back as the DMZ (Port 1, 2, 3, etc.)
\Netgear WNDR4500. Since I upgraded, I have found the the telephone led on the Obi will go out every now and then