I think it's the app "calling you", not an actual phone call.

pimlottc · on Feb 14, 2017

Okay, that makes sense, wasn't completely clear from the video.

Still, they would need to somehow verify it's the right conference call, and not one set up by a phisher.

darkstar999 · on Feb 14, 2017

If it's anything like Slack, the call can only originate from within the application (which is isolated to the single business), so there is no chance for outside calls.