You can participate in a direct conversation / chat with a customer only if you are in a Customer Service Window. Which means the customer should be replied to your template message within the last24 hours. If the customer's last message is before 24 hours and if youtry to send a normal message to the customer it won’t be successful.If you need to resume a conversation with that customer you have to send a template message to the customer and he/she should reply toyou. The direct messaging supports the following message types: Text message, media messages, contacts and location messages and interactive messages.