Open the Phone app
Tap Recents
Tap the info icon next to the unwanted number
Tap Block this Caller
Go to Settings
Tap Phone
Turn on Silence Unknown Callers
Go to Settings
Tap Phone
Tap Call Blocking & Identification
Enable any call-blocking apps you have installed
Go to Settings
Tap Messages
Turn on Filter Unknown Senders
Contact your carrier to enable spam call protection
Use Do Not Disturb or Focus mode to allow calls only from contacts
Report spam calls to your carrier
Delete and block repeated unwanted numbers