Caller ID

1. A telephony network service[?] that transmits the caller's telephone number to the called party[?]'s telephone equipment during the ringing signal or when the call is being set up but before the call is answered.
2. A device for displaying the telephone number of a telephone caller to the called party[?] before they answer the call, so that they can be informed of who is calling.

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Caller ID is also known as Calling Line Identification when provided via an ISDN connection to a PABX.

The Caller ID number display can often be prevented from displaying the caller's number, or "blocked", by the caller if they dial a special code before making a call or as a permanent service on their line.

Telemarketing organisations often block the display of their calling numbers. Some states and countries require telemarketers to display a contact number that can accept complaints, as the individual caller numbers may not be able to be called.