Ah. The phone plugs into an FXS port, not FXO ports. FXS (Foreign eXchange Station) means “plugs into a device” and FXO (Foreign eXchange Office) means “plugs into a POTS line.” At one end of an analog connection is an FXO and at the other is an FXS. Meaning, the phone plugs into an FXS jack which eventually connects to an FXO port that the phone line plugs into.
If you want to plug analog phones into an HT814, that’s fine; you have 4 FXS ports so you can plug in up to 4 analog devices (phones, FAXes, whatever).
My confusion comes from when you switch and start talking about “lines.” Those are typically POTS analog wires that come out of the phone company’s central office, traverse dozens of miles of analog copper on a telephone pole, and then end at a jack on your wall that you then plug into your phone system or phone. THIS part, through the magic of two 2-pair wires in the jack, in a duplex arrangement. And it’s THIS part that I’ve never seen a VoIP FXO port handle as a duplex connection.
Again, you can take one duplex RJ11 connector, cut it off, split the pairs, and recrimp them on two separate RJ11 connectors if you need to split a duplex line. Assuming you have a duplex tool and some RJ11 connectors, it will take about 60 seconds.