Ask clearly and keep a record
Explain what messages someone will receive and why before collecting an opt-in. Record when and how permission was given, and keep that record associated with the contact. A phone number on its own is not proof that the person agreed to marketing.
Respect platform and local requirements
Use the appropriate business product and approved provider setup for the use case. Follow current WhatsApp rules, template requirements and applicable local privacy and telecom requirements. Keep messages relevant and avoid purchased, scraped or borrowed contact lists.
Make stopping easy
Provide a clear opt-out route, apply it promptly across relevant lists and limit access to contact data. Review delivery complaints and update the process when policies change. See the current WhatsApp Business Messaging Policy; this guide is not legal advice.