Here they are:
- First, you need to access the list of recent calls by opening the Phone app.
- Now tap on the Recents tab at the bottom.
- Click the "i" symbol near unwanted phone number
- Scroll all the way down and select Block this Caller.
- Just confirm your decision
By the way, the owner of the phone number is never notified you have blocked them. Blocking contacts is a one-way street: you can still call, text, or FaceTime the numbers you've listed in Blocked.