I know the U-boat bridge had some sort of voicetubes so the watch could communicate with the below deck crew. But was the communications two-way? I'm assuming it was, but can't ever recall an example. How did it work? Was there a buzzer, bell, light or some type of signal that told the bridge to "answer the phone." How did the control room communicate with the bridge without shouting up an open hatch? I know it seems like a small point...but an answer would be much appreciated.