When a customer messages you, a 24 hour timer called a customer service window starts.
If you are within the window, you can send free-form (+ templated free form) messages or update template messages (at additional cost). If you are outside the window, you can only send update template messages.
💡 Since November 2024 service conversations all are free of charge across all of WhatsApp Business Accounts and their business phone numbers.
Marketing, utility and authentication conversations are not part of the free tier. (Source: Meta)