Opt-in is the explicit consent customers give to receive SMS messages. It's legally required for marketing and a best practice for all SMS communication. Types of opt-in: - Single opt-in: Customer submits phone number - Double opt-in: Confirmation text required - Explicit opt-in: Clear consent language Opt-in requirements: - Clear description of message types - Expected frequency - Standard rates disclosure - Opt-out instructions - Privacy policy link Collecting opt-ins: - Website popup or form - Checkout checkbox - Keyword texting (text JOIN to...) - Point-of-sale - QR codes