Unknown callers are identified by carriers when network data fails to provide a number, while No Caller ID indicates the caller intentionally blocked their identity using features like *67.  Carriers can trace these numbers if the service originates from a domestic network, often sending warnings to the originator, though international masked calls are generally untraceable by providers.  Third-party apps and reverse lookup tools attempt to identify these numbers by scraping public databases and social media, though success is not guaranteed for spoofed or unlisted lines.  Users can enable built-in smartphone settings to silence or block unknown callers, sending them directly to voicemail to avoid potential fraud and wasted time.

Kylesinvestigation.com