Neckloop A handset coupler is a device that goes between the deskset of a telephone and the handset. It has a modular plug on a cable that plugs into the deskset. The cable from the handset plugs into it. The signal to and from the handset is not altered in any way, so people with normal hearing can use the handset just as if the coupler is not there. The coupler taps off the signal to the earpiece and routes it to a cable or jack where an ordinary amplifier is connected.
Some handset couplers have an amplifier built in, so only a neckloop is required.
This arrangement works very well on analog residential phones and on most digital phones too because the signal at the handset plug is just ordinary analog audio. It does not work on one-piece phones, cordless phones, cell phones, or pay phones. There may be others. It has equalization controls and a headphone jack, so I assume a neckloop can be plugged into the headphone jack. I need to research this further and expand this description, but in general, bigger looped areas require more amplifier power.