Nobelphone field telephone

[dsk]

I have 2 of these field phones, strong sound, terrible howling in the earpiece (in both phones) as a callsignal.
Maybe a transistorized version of the Stanofone? 
Still made? http://www.nobelphone.com/english.html

[countryman]

It would be interesting to analyze the circuitry. Why 4 (?) transformers? Piezo Elements ?for transmit and receive?

[dsk]

Now the less used has stopped working. No leak from the batteries marked 76.

[dsk]

It would be interesting to analyze the circuitry. Why 4 (?) transformers? Piezo Elements ?for transmit and receive?

Changing the piezo elements from the defect phone to the other and the defect works, but not the other...
I do not understand anything here, but here are some pictures of the circuitry.

[countryman]

It looks like a toy at first glance but apparently is a quality product made in Japan. Google Chrome's translation of the mostly Japanese website gives a rough idea, but not information about the working principle. I can see only 2 traditionally wired transistors and the aforementioned 4 transformers per set. The smaller metal boxes presumably are the switches for calling and speaking.
I can only guess the transistors will generate a howling call tone and provide amplification for speaking - the energy from a piezo transmitter is too small to allow direct communication.
Siemens used piezo elements in some 90ies phones. Possibly these might be compatible. There is a small market for piezo powered phones, because allegedly they do not emit electro-magnetic fields like dynamic receivers do. Saves the user from wearing a tinfoil hat all the time