pc44,
Google Voice doesn't use SIP ports, so forwarding 5060 - 5061 won't help Google Voice. I'm a believer in forwarding them for SIP providers, but it's not always necessary.
Google Voice uses RTP ports for audio, just like SIP providers, but since that thread was active, we've learned that the OBi doesn't appear to honor the specified SIP port ranges when it comes to Google Voice. I don't recall anyone ever figuring out if there is a defined RTP port range for Google Voice. Obihai is always quiet when these questions are asked.