Do MultiLine users have to initiate chats with WhatsApp users? What happens if WhatsApp users message first?

MultiLine users must initiate the messaging conversations with WhatsApp Users. This results in the WhatsApp Business Messaging opt-in flow

If a WhatsApp user sends a message to the enterprise WA Business number first, that user will be notified that that they can't message that number.  A custom template can be created when the organization enables WhatsApp Business, that will send the user a message with a number the person can message using SMS.