Plug the short 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 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 a Straight Handset Cord to the Handset 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 the Handset Y Adapter on your voice logger, use the Deluxe Handset Recording Adapter which is transformer isolated.
NOTE: Some poorly designed phones don't have sidetone, which means you can't hear your own voice in your ear when you're speaking. On those types of phones, any Handset Recording Adapter will only record the outside caller (half the conversation).