Caller ID (caller identification, CID), also called calling line identification (CLID), (CND), calling number identification (CNID) or calling line identification presentation (CLIP), is a telephone service, available in analog and digital phone systems and most voice over Internet Protocol (VoIP) applications, that transmits a caller's 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.

Caller ID may be used by the recipient to avoid answering unwanted incoming calls by the concept of informed consent.

However, it also poses problems for personal privacy.

Usually, the caller can disable or "block" the Caller ID facility, but some call capture technology can bypass a caller block.

On the other hand, the possibility of caller ID spoofing may render received information unreliable.

A telephone number is a sequence of digits assigned to a fixed-line telephone subscriber station connected to a telephone line or to a wireless electronic telephony device, such as a radio telephone or a mobile telephone, or to other devices for data transmission via the public switched telephone network (PSTN) or other private networks.