This action is supported on phones which do not have a permanent Drop button.
For a currently connected call, pressing Drop disconnects the call. When drop is used to end a call, silence is returned to the user rather than dial tone. This is intended operation, reflecting that Drop is mainly intended for use by call center headset users.
If the user has no currently connected call, pressing Drop will redirect a ringing call using the user's Forward on No Answer setting if set or otherwise to voicemail if available.
For a conference call, on phones with a suitable display, Drop can be used to display the conference parties and allow selection of which party to drop from the conference.