whose-number-is-this

# Whose number is this A phone number is a strong selector: expensive to acquire, hard to change, and used as an account-recovery anchor almost everywhere. It is also the selector investigators get most wrong, because the metadata everyone quotes — the carrier — is frequently stale, and because a large share of numbers now belong to software rather than to a SIM in someone's pocket. Do not call or text the subject; that is contact, not research. ## Step 1 — Authorized scope Read [../../ETHICS.md](../../ETHICS.md). Write down subject, objective, in-bounds selectors, out-of-bounds actions, and the governing jurisdiction — which matters more here than in most skills, because reverse phone lookup is specifically regulated in many places. Decide now whether messaging-app contact-discovery checks are authorized; they are interactive. **Done when** scope is written and the contact-discovery decision is recorded. ## Step 2 — Normalise to E.164 first Everything downstream breaks on a badly parsed number. E.164 is `+`, country calling code, national number — no spaces, punctuation, or trunk prefix. The trap is the national trunk prefix. A UK number written `07700 900123` is `+447700900123`: the leading `0` is dropped, not kept. Italy keeps its leading zero for landlines. Guessing wrong shifts every subsequent lookup onto a different subscriber. Use `libphonenumber` (Google's parsing library, with bindings for most languages) rather than a regex. It knows per-country trunk prefi...