Open the Settings app
Tap Phone
Turn on Silence Unknown Callers
Unknown numbers not in your contacts, recent outgoing calls, or Siri suggestions will be silenced and sent to voicemail
To block a specific number, open the Phone app
Tap Recents
Tap the info icon next to the number
Tap Block this Caller
Confirm by tapping Block Contact
To manage blocked numbers, go to Settings
Tap Phone
Tap Blocked Contacts
Add or remove numbers as needed