SMF is used to pass through calls/messages when device is muted. It can be used to replace a call blocker. There are three modes:
Select any mode other than "Off". Whenever you mute your device (using power button) the filter turns on.
Your phone has to be in silent mode (not vibration mode). It may be disabled by default on some devices: go to Android sound settings and set vibration to "Only when not in silent mode"
SMF uses alarm audio stream. Alarm audio stream must be enabled when your phone is muted. It may be disabled by default on some devices: open Android alarm application and check "Alarm in silent mode" option. If it cannot be enabled, go to application settings and try switching to music audio stream.
Use it if you wish to be notified about SMF activation/deactivation when screen is locked. Go to settings and check "Silent Mode Filter popup" option.
Call blockers, apps that control/mute volume or SMS apps can affect other apps on your device (some of them "swallow" incoming SMS messages before other apps can catch them). If SMF does not work, try switching to Android default SMS app and turn off all volume control and call blocker apps.