A button set to this function allows the user to lock their extension from being used to make calls. After they press the button, they are prompted to enter a four digit code after which the extension is locked. If the extension is already locked, pressing the button prompts for reentry of the four digit code to unlock the extension.

Any locked extension can be unlocked from either of the first two extensions in the system without needing the four digit locking code using a Station Unlock button.

If an extension already has a button set to this function, creating another button with this function automatically clears the setting from the existing button.

To access this function without a programmable button:

On DS and ETR phones, press FEATURE and then dial 21.

On BST phones, press FEATURE and dial 936.