Masks the display of digits as you dial them on the phone. Dialed digits are replaced with an s character. The button is used to switch the suppression on/off. When 'suppress digits' has been set, the button's red lamp is lit. Pressing the button again cancels the suppression.

If en-block dialing is active you will see the digits as you dial them. When you make the call the digits will be suppressed.
When the option to suppress digits is used, the call is not recorded in the call log.