Can third parties listen to phone calls in a conference?

What "3rd parties" are you talking about? What's your real concern?

Have you read this support article?:--> How to make a conference call from your iPhone - Apple Support (CA)

Regards

Giulio

If someone on the conference call has the phone on speaker and there's someone else in the room. sure. Or if whoever originated the call also brings in someone other than you, yes.

But as ProustGiulio says, you really need to give us more information.