Open the Phone app
Tap Recents
Find the number you want to block
Tap the info icon next to the number
Scroll down
Tap Block this Caller
Tap Block Contact to confirm
Open Settings
Tap Phone
Tap Blocked Contacts
Tap Add New
Select the contact or number to block
Open Messages
Open the conversation with the number
Tap the contact name or number at the top
Tap Info
Tap Block this Caller
Open FaceTime
Tap the info icon next to the number
Tap Block this Caller
To unblock, go to Settings
Tap Phone
Tap Blocked Contacts
Swipe left on the number
Tap Unblock