A user can join an ongoing audio/video call anytime while it lasts (unlike with standard peer-2-peer calls) as long as they are listed in the Occupants_ID list of the room/dialog.
If a user loses connection during a conference call, as long as the room/dialog is persistent, the application will automatically try to reconnect them. It is also possible to implement a rejoin button.
You can check if the call is still on using a code snippet provided here: List online participants section.
Feel free to check our official documentation for more information that will help you to integrate QuickBlox services into your applications / projects or contact our Sales team by submitting a ticket if you have any other questions.