Converting a 500 phone to "field telephone" ?

[dsk]

Hi
I'm trying to make a description so simple as possible about how to redesign a 500 set to LB with howler signalling like what e.g. the Brits used on their field telephones.

The 2 telephones are typical British made field telephones. I do actually not like this howler idea, but the lack of magnetos made this solution, and it is so simple, and still reliable.  I made this text to the enclosed pictures, so if you have ideas of how to improve it with extremely simple modifications, pleas post it here. 

Text:

An old rotary telephone as a field telephone? 

DIY projects use to generate more work than expected, this one might do so too, but since
field telephones tends to be rare, and expensive so might this be an example of how you may
convert an old rotary to work as a kind of field telephone, and with a range of at least 10
miles.

I have based this on the most common rotary telephone in North America. It looks equal, and
are made by lots of makers, but use to be identical.

The main problem will always be the lack of the hand crank generator, so let us skip that, and
do as the Brit's did, make it howl.

It may be harmful to have the receiver to your ear when howling, but that is equal for the 2
green phones in picture. The black one is the one I modify, and in the schematics, I have put
in pictures of how the components may look in real life, not only as a schematic symbol. This
should make it possible to do the job with no knowledge about electricity. The batteries
needed will usually be any battery with a total voltage between 2 and 5 volts. 
The coil used may be any coil that works, typical a relay with high number of winding's, but
low resistance. You have to try out among what you have laying around. 
The buzzer has to be the old fashioned version, it makes an awful noise, and made for
working on such lo voltage as e.g. 3V. 
The switch has to return to microphone circuit when released. (spring loaded)

dsk