We used to have them on board ship in the Navy for communication between different parts of the ship. They had no power source, so they were resistant to battle damage. No doubt they have been replaced by conventional powered phones now.

You spoke into the phone and your voice moved a diaphragm which moved a magnet in a coil and generated a signal current. The guy at the other end had the recieved signal current move an electromagnet in his headset which would vibrate diaphragms in his headphones. The sound was clear, distinct (you could easily recognize people’s voices) and loud. It sounded just like a regular telephone, but your voice provided the energy (of course, the signal only had to travel at most, a few hundred feet).

Sound powered phones worked on ‘telephone nets’
several phones on one line, so you could talk to anyone on the net, but all the others on it could hear you as well. Strict telephone procedure discipline was required to prevent confusion. For example, in a typical conversation between the messenger on the bridge and a phone talker in the main engine room:

“Main control, bridge.”
“Main control, Aye.”
“Main control, Bridge, This is a drill, this is a drill. Stand by for simulated loss of engine order telegraph.”
“Main control, Aye, Lee Helm Casualty drill, Aye”
“Main control, Bridge, Lee Helm Casualty, make turns for one-two-zero RPM, all ahead one third, execute.”
“Main control, aye, one two zero turns.”

The bridge and main control phone talkers were actually relaying commands and responses between the officer of the deck on the bridge and the Duty engineering officer in the engine room.

Every telephone talker shared his line with several other talkers, and some talkers had switch boxes allowing them to switch from one net to another. So for example, the bridge messenger could talk to the gunnery officer, the lookouts, the signal bridge, and so on. The gunnery officer could talk in turn to each mount captain, fire control director, magazine supervisor, and so on. There were color-coded phone jacks all over the ship so even if you were forced to move from one place to another, you could usually find one to allow you to talk to the people you needed to talk with, or at least find someone to relay a message for you.

Without reliance on power, as long as one unbroken wire connected you to someone else, you could usually get your message to someone who needed to talk to you.

I wonder if the SP phone is used in other applications.