Open the Settings app
Tap Phone
Tap Call Blocking & Identification
Turn on Block Contact
Select Contacts to choose a number to block
To block an existing caller: open the Phone app, go to Recents, tap the info (i) next to the number, then tap Block this Caller
To unblock: Settings → Phone → Call Blocking & Identification → Edit → tap the red minus (–) next to the blocked contact or number
To block unknown callers: Settings → Phone → Call Blocking & Identification → turn on Silence Unknown Callers