That's how VoIP works. You have "endpoints" (one IP phone and one ATA) registered to a SIP server (Google Voice). Each registration is independent. Calls between the endpoints and the server are digital. The IP phone does not convert the digital signal to a POTS telephone analog signal. There is no way to connect it to your other phone lines the way you described.
However, you can connect the OBi 200 ATA's phone jack to your house's in-wall telephone wiring, and then plug analog telephones into jacks in other rooms. Or, of course, do what most people do, plug a DECT cordless phone base station into the 200's phone port, and use as many cordless handsets as you wish.
Finally, Google Voice has a feature that will accomplish the same goal: when you answer a call, and you want to transfer it to another Google Voice endpoint (another OBiTALK device, or an Android or iOS app client), you can push the star (*) key. This will cause all the endpoints to ring again, and anyone can pick up the call on another device or app. You'll need to enable "Incoming call options" on the desktop Google Voice website's settings page.