Open the Phone app
Tap Recents
Tap the (i) next to the number you want to block
Tap Block this Caller
Tap Block Contact to confirm
Open Settings
Tap Phone
Tap Blocked Contacts
Tap Add New…
Select a contact or enter the number
Tap Block
Open Settings
Tap Phone
Turn on Silence Unknown Callers
Optional: Turn on/enable Call Blocking & Identification (if available)