Are you getting a lot of calls from unknown callers (e.g., spammers, telemarketers etc) with “No Caller ID”. You may want to block these calls because you cannot possible identify the number of a caller before answering. These calls may also be labeled as Anonymous, Private, or Blocked.
You are receiving these calls because, in few countries, local authorities mandates that cellular carriers allow people the ability to block their Caller ID, hence No Caller ID. Some ordinary people (VIP) may want to block the display of their telephone numbers intentionally because of some privacy concerns.
There are also more and more telemarketers, spammers using No Caller ID because they do not want to be known.
The good news is that you can block specific callers/numbers/caller IDs using third party apps in the App Store.