No. It's a programming item and a code or softkey.

The user connects the modular headset to the handset jack on
the telephone leaving the handset in place. The ON/OFF
button on the Digital Telephone is used to initiate and end calls
while using the headset.
To activate Headset Mode:
Dial [634] on the dial pad,
-or-
Press preprogrammed HEADSET MODE button. LED lights
steady.
To deactivate Headset Mode:
Dial [634] on the dial pad,
-or-
Press the preprogrammed HEADSET MODE button. LED
extinguishes.

While Headset mode is active, the ON/OFF button
activates the headset and disables speakerphone
and intercom call announce operation at your
station.
To install the headset, see the Installation Manual.
Station must be programmed in database
programming for headset operation before the
flexible button can be programmed.

Carl