Oftentimes, HP fax machines have 2 ports in the back. One may be labeled LINE and the other perhaps EXT or Phone.
Normally, the line from the wall jack would connect to the phone companyt and the phone plugs into the other port, putting the fax machine between the phone company and the phone. This allows the call to come in, the fax machine to be faxing and the machine would disable the phone so that picking up the phone would not interrupt the call. HP machines may send you a special line cord that has only 2 wires.
So with VOIP, you could duplicate the same setup by plugging the line code from the fax into the phone port of the Obi.
As mentioned in another reply, you can use a two way splitter if you want, plugging the male end into the phone port of the Obi and plugging a phone in one of the two ports and the fax machine in the other.