Turn on Silence Unknown Callers to avoid getting calls from people you don’t know. This feature blocks phone numbers that you've never been in contact with and don’t have saved in your contacts list. If you have once texted with someone using their phone number or if a person has shared their phone number with you in an email, the call will go through.
To enable this feature, go to Settings > Phone, scroll down, then select Silence Unknown Callers. Calls from unknown numbers are silenced and sent to your voicemail, and appear in your recent calls list. This feature is only available in iOS 13.