WhatsApp Business API has what is called 24-hour window limitation. This limitations basically means that you as a company has just 24 hours to reply to a customer’s messages after it was sent to you via WhatsApp. Once a customer sends you a WhatsApp messages, the 24-hour session is started. If you do not respond on time, you cannot continue a normal conversation with the customer as your dialog becomes locked. To send a new message to such customer, you can only use a Template message that was submitted to and pre-approved by Facebook. Keep in mind that such template message cannot be of a marketing or sales nature.