There's no way to signal the operator, but what you can do is to use the in-call menus to unmute yourself - press * to enter the menu, then 6 to unmute the audio. The participant can either mute themselves again when they've finished speaking or else the operator can do it.
Perhaps a simpler option, however, would be to mute the microphones on the individual endpoints, and then have the user unmute themselves when they want to speak simply by pressing the unmute mic button on the endpoint/remote/microphone as appropriate - this would seem to be a more logical, natural and user-friendly way of doing it, with the least user education.
