Are you able to share any of those security controls? How do you stop presumably well-intended Signal app users from conferencing? Are you talking about cellular signal blocking, or are you talking about avoiding public networks entirely in favor of Sensitive Compartmented Information Facilities (SCIFs)?

Many layers of physical controls and regular audits mostly.

So in other words... nothing stopped anyone from doing this either. Except the fear of potential punishment.

I'm not sure how you ignored the "many layers of physical controls" part of the comment.

What does a physical control mean in this context? Like, disabling that part of the phone's touchscreen?

Think personal devices not being permitted beyond the entrance of the building, stored in a Faraday box.