To silence the ringer of an incoming call, you can press the “Power” button. On some phones, pressing the “Volume” button will also switch the ring to silent mode. Additional methods are available on other phones. When the phone is in silent mode, the call will continue to ring until it is ended or forwarded to voicemail. The caller will not be aware of your action.
Note that this only silences the ringer of the particular incoming call.
Alternatively, you can dismiss/reject/decline the call or set to vibration mode by adjusting the volume to minimum.