How to Block Calls from Unknown Numbers on an iPhone
To block calls that come up as "Unknown" on an iPhone, start by opening your phone app. After you get into your phone settings, tap “Contacts” and the plus sign in the upper-right corner of the screen. Type “unknown” into the first and last name fields, then click “Save” in the upper right corner of the screen. Finally, tap “Block this Caller” at the bottom of the screen. You can also block calls from numbers you don't recognize by going to “Recents” in your phone app and tapping the blue icon on the right side of the number you don't recognize. Scroll down and tap “Block this Caller” at the bottom of the menu.
Open your iPhone's Settings. It's a gray app containing gears on your iPhone's home screen. Tap Do Not Disturb. It's in a section near the top of the menu, next to a purple icon with a moon inside it. Tap Allow Calls From. It's midway down the screen Tap All Contacts. It's located in the "Groups" section of the menu. Now, when "Do Not Disturb" is enabled, only numbers in your contacts will get through. Swipe up from your home screen or lock screen and tap the round, crescent moon icon at the top of the Control Center to turn Do Not Disturb on or off. Open your Phone app. It's a green app in the lower left corner of your iPhone's home screen. It contains a telephone icon. Tap Contacts. It's in the bottom center of the screen and contains the silhouette of a person. Tap +. It's in the upper-right corner of the screen. Type "Unknown" into the first and last name fields.  Tap Save. It's in the upper right corner of the screen. Tap Block this Caller. It's at the bottom of the screen. Tap Block Contact. Now, most calls labeled as "Unknown" should be blocked from your iPhone. Friends calling from an unknown number will not be able to reach you. Open your Phone app. It's a green app in the lower left corner of your iPhone's home screen. It contains a telephone icon. Tap Recents. It's a clock icon in the bottom left corner of the screen. Tap ⓘ next to a number you don't recognize. It's a blue icon on the right side of the screen. Scroll down and tap Block this Caller. It's at the bottom of the menu. Tap Block Contact. Now, calls from this number will not reach your iPhone. 
Open your iPhone's Settings. Tap Do Not Disturb. Tap Allow Calls From. Tap All Contacts. Open your Phone app. Tap Contacts. Tap +. Type "Unknown" into the first and last name fields. Tap Save. Tap Block this Caller. Tap Block Contact. Open your Phone app. Tap Recents. Tap ⓘ next to a number you don't recognize. Scroll down and tap Block this Caller. Tap Block Contact. 