When you receive a phone call that says "No Caller ID" on your iPhone, it means that the number was purposefully blocked and hidden before calling. This is different from an "Unknown Caller," which means that the caller ID was not recognized even though it was provided. "No Caller ID" calls are from callers who overtly took extra steps to hide their number from you, so you cant see whos calling, and most likely arent calls that you need to answer. Its important to note that emergency services can still trace the call even if the caller ID is blocked. If you want to block unknown callers on your iPhone, you can turn on the option to do so in your phone settings.