The manufacturer of this phone is unknown, could be made by VR (Valtion Rautatiet, State Railroad here in Finland), because their have used it, text can be found in the crank. On the back there is stamped number 522.
Part's are taken from Siemens made Army field telephone used in WWII.
The holding part of the handset has been covered with two parts of birch (push button removed) and also receiver cover made from birch, both have been painted black.
I think the parts have been taken from this kind of field phone.
Date unknown, but made by Siemens.
Picture taken from old phone display of SSP (local phone operator here in Salo).