CLI

Calling Line Identification (CLI) is the transmission of the caller's phone number to the receiving end of a telephone connection. The exchange sends this data as a separate signal, usually in-band on analogue lines or via the signalling channel on digital circuits, arriving before or during call setup. On the receiving end, equipment decodes and displays the number, allowing the called party to see who is calling before answering.

CLI operates differently depending on the network type. On traditional PSTN lines, the number arrives via Bellcore FSK (frequency-shift keying) in North America or DTMF tones in some other regions, transmitted after the first ring. On ISDN and modern VoIP systems, CLI travels in the Q.931 call control signalling, making it available before any ringing occurs. Digital systems therefore allow calling number display before pickup, while older analogue systems show it between rings or after answering.

A critical distinction exists between CLI and ANI (Automatic Number Identification). ANI is what the telephone exchange itself knows and uses for billing and call routing. CLI is what the exchange chooses to send to the far end. These can differ: a switchboard may send a main reception number as CLI rather than the individual extension, or a caller may have requested CLI suppression (sometimes called "withhold" or "private number").

In telephony installations, CLI appears on handset displays, in call centre computer telephony integration (CTI) systems, and in logging equipment. Problems arise when CLI is absent, delayed, or incorrect. Poor signalling conditions on analogue lines can garble the FSK burst. Digital PBX systems may be misconfigured to withhold or substitute the CLI. VoIP systems may send a number that does not match the calling party's actual line, creating confusion or spoofed caller displays.

CLI is governed by national telecom regulations. Many countries require carriers to provide it by default, with explicit opt-out available for certain callers. Some jurisdictions mandate CLI presentation in emergency calls (e999 services) regardless of suppression settings. Equipment standards, such as ITU-T Q.931 for ISDN, define how CLI data must be formatted and transmitted across network boundaries.