Plug the short 4 Inch Cord on the Handset Y Adapter into the handset jack on the phone, and then plug each handset (with cord) into the jacks on the Handset Y Adapter.
We also make Push-to-Talk Handsets (and Modular Mute Switches that you can use with any handset) that you could use as the second handset for a supervisor who primarily needs to listen - not talk.
GREAT for training:
You can connect 2 handsets to a single phone or a combination of handsets/headsets. Handsets and Headsets sold separately.
GREAT for use with Voice Loggers:
The Handset Y Adapter can also be used to send the nicely volume balanced handset audio to a central Voice Logger for recording by connecting aStraight Handset Cord to the Y Adapter, which goes to a jack on the wall, which sends the audio back to the phone room - and then to the Voice Logger.
If you hear hum using this the Handset Y Adapter on your voice loggger, use the Deluxe Handset Recording Adapter which is transformer isolated.