Here they are:

  1. First, you need to access the list of recent calls by opening the Phone app.  
  2. Now tap on the Recents tab at the bottom.
  3. Click the "i" symbol near unwanted phone number 
  4. Scroll all the way down and select Block this Caller.
  5. 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.