How to use a DKO48 telephone microphone?
IIRC almost all microphones used in telephones were carbon. If this is a carbon microphone (shake it and you should hear a noise of the carbon powder moving around) then you should connect it in series with a speaker and a DC power supply. That's it.

The microphone basically acts like a variable resistor.

If you want to use it to record something (that is, not just for connecting to a loudspeaker), you need to connect it in series with a resistor and a power supply, like this:


To protect your equipment from DC offset, connect a small capacitor (10uF or so) in series with the "+" wire.