What is CLI (Calling Line Identification)?
CLI (Calling Line Identification) is the telephone number presented to the called party as the origin of a call — the voice-network equivalent of an SMS sender ID. It is carried in call signaling (SS7 IAM, or SIP From / P-Asserted-Identity). CLI can be legitimately set — a business showing its main number — or fraudulently spoofed, which is exactly why authentication frameworks like STIR/SHAKEN exist. See /answers/what-is-ani, /answers/what-is-stir-shaken, and /answers/what-is-did.
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