The Feldfernsprecher 33 (FF33) was designed to be weatherproof. As such, the FF33s were constructed with rubber seals between the bodies and the lids. The seals allowed the field telephone wires and the handsets to connect with the units with the lids closed. The rubber sealed the interiors from rain.
Although it looks cool to leave the lids open, this is not how the phones were typically used in WW2. The lids were closed when the phones were used inside and outside. When the lids were closed, the handsets were placed on the lids.
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WW2 German Field Telephone Equipment 