speaking tube

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A tube used to convey the voice over a short distance, as from one part of a ship to another.

from The Century Dictionary.

• noun A tube of sheet-tin, gutta-percha, or other material, serving to convey the voice to a distance, as from one building to another, or from one part of a building to another, as from an upper floor to the street-door, or from the rooms of a hotel to the office.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a tube for conveying the sound of a voice from one room to another