iOS 13 has a new feature that may protect you from spam and robocalls only. The new feature "Mute unknown callers" mutes all calls coming from a number that is not in your contact list. Your contacts and people you interacted with can still call you, but you will not be disturbed by incoming calls from other people.
This feature is one of the main reasons why we're looking for iOS 1
How Mute Unknown Callers Works
The Mute Uncalled Caller feature is a pretty one clear instrument – it's a simple switch in the settings. Once activated, any call you receive from an unknown number will be muted. When Siri Intelligence finds the number in the mail, message, or contact app, the call is put through.
You can imagine that iOS automatically presses the Reject button for you. You will still receive a missed call notification and find the number in the Recent Calls list in the phone app.
If you have voicemail enabled, the call is automatically forwarded to voicemail. Here you can use the visual voicemail feature (availability and fees depend on your mobile service provider).
When you click the Voicemail tab in the Phone app, you can listen to the transcript or read the voicemail (as you can see in the screenshot below).
If the message is important, you can call it back. If not, delete the voicemail and lock it if necessary.
In our tests, we've found that any subsequent calls you receive from the number will not automatically mute when you call back the number.
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Enable Silence Unknown Callers
This feature is not enabled by default. To activate it after upgrading to iOS 13, open the Settings app and go to the Phone section.
Scroll to the bottom of the page and tap the switch next to Mute Unknown Callers to turn it on. 
feature To make sure you do not miss any important calls
The Mute Caller Alerts feature has few features and no control. To ensure that you receive incoming calls from someone else, add the person's phone number to your contacts. That's all you can do to control it.
Unlike the Do Not Disturb feature, a call will not be put through after repeated attempts. When this feature is on, your iPhone will not ring even if you're called 6 times in a row.
If you are concerned about missing an important call or emergency call from a pay phone or hospital, you should not do so. Enable this feature.
However, you still receive voice mail from unknown callers. As long as you check your voicemails, you will not miss anything important.
How to Check Voicemail for Unanswered Calls
Some of these unknown calls may be important, and hopefully these important callers will leave a voicemail for you. You can retrieve voice mail from unanswered calls through the phone app.
If you have Voicemail enabled, open the Phone app and go to the Voicemail tab. If you have not activated it yet, you can do it from this screen.
Voicemails from callers are displayed at the top. Tap a voicemail from the list or select the appropriate "i" button to expand the voicemail.
Tap the "Play" button to listen to the message. If you have activated the Visual Voicemail function, a section of the transcribed text is displayed below the number. Touch the field to expand the text.
Alternatives to Silencing Unknown Callers
If the silent blocking of most of the world's phone numbers sounds too aggressive, there are a few alternatives that can be used to mute spam calls.
Although this is not exactly the same thing, you can set the Do Not Disturb feature to allow only calls from Favorites. DND also provides the ability to allow repeated calls. This means that a second call from the same number will not be muted within three minutes.
You can also use a spam blocking app like Hiya to automatically block spam and robocalls. However, this is not a complete solution. The success rate of the app depends on the database of spam callers, which is never complete. Although not all spam calls may be blocked, most should still be blocked.
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