Using the orginal hand-set microphone and speaker...

Up to now I have used the Nokia hands free kit, plugged into the phone then concealed in the old phone handset, bit of a cheat!I then began to wonder if the original hand set mic/ear would work.

I purchased a 2.5mm ear/mic jack plug from ebay, soldered 4 wires to it - which was very fiddly (this is for a stereo connection which wont be needed) connected it to one of the other old handsets I had, plugged it into the Nokia and bingo! Made a call. The 40 year old speaker was deafeningly loud, which surprised me.

 Nokia hands-free cheat

 

The old hand set wired up to the Nokia jack plug.

Check this reference!
http://www.samhallas.co.uk/collection/bits_hs3.htm
This brown hand-set is form the 70s, the cream phone I have used is from the 80s, one of the differences being that the internal microphone is different. When I connected the 80s handset the Nokia would not recognise as a "hands-free" kit which it needs to in order to work. So I swapped the mic from the older phone to the newer one and it worked fine. So either the new version of the mic is faulty or there is something about the older mic that is recognised by the Nokia. I will take some impedance measurements, or look on Wikipedia and see if I can find out.