You can offer video calls without scheduling times in advance by receiving notifications when users join a room. To do this, you'll need to configure a webhook for individual rooms or your entire subdomain using the Daily.co API. You can create a room and set its meeting_join_hook property to receive POST requests with information about the room and who joined. You can then use these notifications to initiate actions in your application, such as notifying your support team that there is a user waiting. Additionally, you can configure domain-level notifications for all rooms in your subdomain, which can be useful for implementing usage tracking or responsive room management.