Panasonic KX-TGM420 Operating Instructions Manual page 66

Setting the headset type
Set this setting to match the type of device connected.
This feature can be set for each unit.
"Headset" (default): Disables the handset's
microphone. The connected headset is used for talking
and listening.
"Neckloop with Mic": Disables the handset's
microphone. The connected neckloop is used for
talking and listening.
"Neckloop w/o Mic": Enables the handset's
microphone. The connected neckloop is used for
listening only.
1 [MENU] [#] [7] [8] [6]
2 [ ]: Select the desired setting.  [SAVE] 
[OFF]
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