A way to validate whether a guest's phone number is real, usable as an automation condition. OTA bookings carry dummy numbers (Expedia all-1s, Booking.com reference numbers), so 'has a phone?' isn't enough - this breaks the email-fallback automation (#33). Options: pattern checks (known OTA placeholders) as a quick win vs. carrier lookup (Telnyx). New condition with three states: valid / invalid / missing.