Samsung SGH-ZX20ZKACIN - Manual (Page 33)

Section 3: Call Functions 33
In Silent mode, instead of sounds, your phone will vibrate to inidicate activity, such
an incoming call or message. It will also turn off the sound made when keys are
pressed.
To exit and reactivate the previous sound settings, press and hold the key
again. The Silent mode icon ( ) no longer displays.