Well, for starters, it has nothing to do with Java; nothing used by OBiTALK or Google Voice uses Java. Why did you think that mattered? If you've blocked various ports, or blocked javascript, etc, then that could be an issue.
Try a different web browser (Firefox, Chrome, Safari or MSIE) than the one you are using. Try using private or incognito browsing mode.
It's possible that the device isn't "factory fresh", but somebody bought it, fiddled with it, added it to their account, didn't want it, and then returned it. If so, then it may be stuck on somebody else's account.
While powered-on, but with the Ethernet cable detached, use a paper clip to gently depress the recessed reset button on the underside of the device for at least 10 seconds, to initiate a factory reset. After it completes, you should be able to pick up the telephone plugged into it and hear a dial tone. Attach the Ethernet cable and wait a couple minutes. Press ***1 to ensure the device is getting an IP address assigned.
Now, log into the OBiTALK portal, and use the **5 procedure to attempt to add it again. If it fails, follow the troubleshooter, here:
http://www.obihai.com/support/troubleshooting/sg/adddevIf that doesn't work, open a support ticket with Obihai.