https://developers.viber.com/customer/en/portal/articles/2541267-callbacks?b_id=15145#conversation-started Conversation opened event fires when a user opens a conversation with the PA for the first time (no conversation history with the PA), or when the conversation is opened using a deep link (See Deep link section for more information).

https://developers.viber.com/customer/en/portal/articles/2632255-send-message?b_id=15145#send-message-response.