How to Write WhatsApp International Phone Number Format: Plus Sign, Country Code, Leading Zeros, and Special Rules for Argentina and Mexico

How to convert a mixed-source customer list into WhatsApp-compatible international numbers: determine each number's country, strip local leading zeros and 00/011 prefixes, apply E.164 length and character limits, handle the United Kingdom, Argentina (+54 9), and Mexico (+52 1) special rules, and strip to plain digits for APIs and links, with a pre-delivery checklist.