As far as GV is concerned the fax machine is a phone.
As far as connecting the fax machine to the OBi is concerned, the OBi regards the fax as a phone. (There may be some later config changes advisable to optimise fax transmission/reception.)
As far as the fax machine is concerned, the PHONE socket on the OBi is a POTS line socket. Consult the fax manual for how to wire it up.
You must connect a phone or some means of dialling the **5xxxx code to register the OBi with the portal dashboard. You won't need the phone plugged in to the OBi once registered.