Why do they unscrew the phone mouth-pieces?

2018-04-25 08:35:51

In All The President's Men, there's a scene where someone is repeatedly calling The Washington Post, to deny something they've done (or rather, to deny that they did it whilst in the appartment of a woman who's not his wife) and a bunch of people want to listen in on the phone call.

So they do the famous "everyone join the line together so they can all hear".

But before joining the line, some of them appear to unscrew part of the phone. What's up with that?

I assume from context that it's something about not being heard?