- 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.
- 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.
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.