If you can make and receive telephone calls through the OBi using Google Voice, all that's left is to connect your TiVo's telephone jack to the OBi's PHONE Port and see if it works. Modem and FAX operation over Internet telephony services is very hit and miss, so I wouldn't place a bet on either success or failure. You can look at the OBi's Call History to verify that the calls are being made.
If it doesn't work, you should be able to use connect the TiVo to your Internet connection and receive Guide information and updates without using the telephone modem.