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 choose "Don't interrupt" for Sound and notifications interruptions, the filter turns on.
SMF uses music audio stream by default, but it can be changed to alarm in application settings. If both alarm and music audio is muted, SMF will not work.
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.