Hidden number

A hidden number is a call whose identity is deliberately concealed — in France, by dialing *31# before the number. The caller shows as "unknown" or "hidden number" on the recipient's screen. The historic legitimate use is personal discretion; its dominant contemporary use is spam and fraud, which has durably poisoned the perception.

Two practical consequences. If you're a business: never call a client from a hidden number — a large share of people no longer answer such calls, and in France masked commercial prospecting has been banned since 2023. If you're the recipient: know that some legitimate customers call hidden (personal discretion), and an overly aggressive filter can lose them — hence the value of a whitelist over systematic blocking.