How do I prevent unknown callers from appearing on my iPhone 13 mini?

The Unknown Callers feature on an iPhone is designed primarily to silence and prevent interruptions from callers whose numbers are not saved in your Contacts. It does not necessarily block or delete those calls.

When this feature is enabled:

• Calls from unknown numbers are silenced, so your iPhone does not ring or disturb you.
• The calls can still appear in your Recents/Call History, allowing you to see who tried to contact you.
• If the caller leaves a voicemail, the voicemail can still be recorded and accessed later.
• This means you do not have to worry about missing an important call simply because the caller's number is not saved in your Contacts.

The purpose of this feature is therefore to reduce unwanted interruptions without completely hiding unknown callers. You can remain aware of calls you may have missed and review the caller or voicemail at a convenient time. This is particularly useful because legitimate callers—such as a doctor, delivery person, bank, service provider, or someone calling from a new number—may not be in your Contacts.

Read this about blocking calls and call lists, but I'd also recommend you turn on Ask Reason for Calling --> Screen and block calls on iPhone - Apple Support

Unknown calls, by the way, will be in your call lists. Turning that on, doesn't mean they didn't call you.