Finally the volume is working again!

I always thought the key on the board itself would be damaged, but it was only this plastic part. The old rocker was probably worn out and has the button no longer reaches right / down.
The rocker can simply open the battery compartment to be replaced - the phone does not have to be completely dismantled!

The original part has a tiny rubber band barbed this spare part also has this rubber band, but without barbs. This is indeed the kept the volume rocker is not optimal in the mobile, but it simplifies the exchange significantly. Hides the rocker under a protective cover, it does not matter anymore.