Open Settings
Tap Phone
Tap Silence Unknown Callers
Turn on Silence Unknown Callers
Open Settings
Tap Phone
Tap Call Blocking & Identification
Add a blocked contact or recent spam number
Open Settings
Tap Messages
Turn on Filter Unknown Senders
Open Settings
Tap FaceTime
Turn on Blocked Contacts (or Block Calls from Unknown Callers, if available)
Open the Phone app
Open Recents
Tap the (i) next to the unwanted number
Tap Block this Caller
Report spam by tapping Report Junk (where available)
Install and enable a spam-caller identification app from the App Store
Enable Caller ID and spam protection (if available in your region/provider) in Settings → Phone