"Tap the main Settings icon when viewing the homescreen, select the Phone option from the resulting menu and tap the Blocked option beneath the Calls section. Alternatively, select either the Messages or FaceTime option from the main Settings menu to access the same Blocked menu offered through the Phone settings.
Tap the blue Add New button and select the desired number you wish to block from the resulting contact list. To unblock a user, simple tap the blue Edit option in the top-right corner of the Blocked menu, followed by the red subtraction side directly beside the user you wish to unblock. Afterward, tap the red Unblock button to confirm the changes."
Open the Phone app at the bottom of the home screen. Click on 'Recents' at the bottom of the resulting screen. You should then see a list of recent calls. Click on the “i” next to the number you want to block. Scroll down to the bottom of the page and click on 'Block this Caller'.