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In many voice telephone networks, anonymous call rejection is a calling feature implemented in software on the network that automatically screens out calls from callers who have blocked their caller ID information.
The caller usually hears a voice message explaining that their call cannot be connected unless they display their number. Or, some networks allow users to forward anonymous calls directly to voicemail.
The service, together with Caller ID, became possible with the introduction of digital switching technologies to landline telephone services, which became widespread in many countries throughout the 1980s and 1990s.
In a classic digital PSTN network it could be implemented directly in software running on local switching systems or, more commonly, as part of a suite of facilities supported by an additional layer known as the Intelligent Network. This allowed more advanced, software based, services to be rolled out in public telephone networks using dedicated intelligent nodes, operating in conjunction with, but independently of switching systems.