You need to disconnect your landline from your house phones and connect it to the LINE Port of the OBi.
You need to connect your house phones to the PHONE Port of the OBi. Make sure your landline is not still connected to your house phones before making this connection.
Configure Google Voice on the SP1 Service.
Set:
Physical Interfaces -> PHONE Port -> PrimaryLine : SP1 Service
All incoming calls will ring the OBi PHONE Port (your house phones).
911 will automatically go out the LINE Port to your landline.
All other outbound calls will go out SP1 to Google Voice by default.
To place calls using the landline instead of Google Voice, dial **8 + number.