What we're trying to say is:
The OBi 200 or 202 that you presumably own has a telephone line jack on its back side. Using a two-wire, RJ-11 telephone cord, plug one end of the cord into the OBi's phone line jack, and plug the other end into the fax machine. If you also want to use a regular voice telephone on that line, get a splitter, as LTN mentioned. Of course, you need to configure your OBi to use either Google Voice or some other service provider, for that line.
Getting fax to work over VoIP is tricky. If you don't have the technical know-how to discuss VoIP settings on your OBi, then don't expect to get it working reliably.