Communication with Call-In partners without going live

We’re going to livestream a show with 16 Call-Ins, which I’d like all to connect and get ready/trouble shoot while a 30 minute countdown video is running before the actual show start. From what I gather I need to assign each Call-In to an individual audio layer which needs to be set to LIVE for me to be able to hear them. But if the livestream is already running because it begins with the countdown video, my conversation with them obviously must not be streamed.

How do I solve this? Can I create another audio mix which excludes the Call-Ins, and then when the countdown video has ended, switch over to program mix in the livestream output audio mix menu?