How do I block unwanted calls on 5C

You can accomplish this a couple of different ways. You can do it in Settings>Phone>Blocked. Or, you can go into the iPhone Recents list, tap on the ℹ at the end of the number you received the call from, and then scroll down and select Block this caller.

If the call does not come with a number in the Caller ID, such as reading Blocked, Private, Unknown, then you will not be able to block the call. It has to have a number to match for the block to work.

You're welcome.