Use of Event Mode for large meetings is a great way to reduce distractions and keep attendees focused on the content being presented. This is accomplished in part by having all attendees not designated as presenters muted by default when they join.
Just like attendees joining via phone or VoIP, in-room video systems will be muted when Event Mode is enabled and they join. Just like when you dial in on the phone, mute/un-mute can be toggled by pressing *7 on the keypad.
In-room systems cannot join as the host, so if the meeting host is joining via an in-room system, they should first join with their laptop from the room, and choose disable audio and video to avoid echo. From their Windows or Mac client, they will be able to enable Event Mode, as well as control the meeting and share content.