The first idea was to attach the handset to my Android phone and use it there.
To find out what quality is possible with such a headset I bought a Nokia handsfree.
Than I started some experiments.
Step 1: The Headset
Then I replaced the original earphone of the handsfree set by the speaker inside the vintage handset.
The sound was loud and clear. :-)
The keyboard of the Nokia mobile was a bit damaged. So I changed my mind and decided to build a new keyboard by my own.