During the war, radio transmissions, and even telephone conversations, could be overheard by the enemy. This prompted the Germans to caution telephone and radio operators that the enemy is listening (Feind hört mit)! Telephone and radio users were warned against speaking directly about vital information that could be used by the enemy.
To save on materials and facilitate field telephone line construction, German troops used single wire and the ground (Erde) where ever possible… Usually well away from the front lines. Trouble is, with the right equipment, the enemy could use the ground to tap into conversations behind the lines.
In the face of this potential problem, the Germans used the Sprechtafel (Speaking Table). The Sprechtafel is a secret form used to assign code words and numbers to locations, unit headquarters, sensitive information, tactical objects, and tactics. These codewords and numbers would be nonsensical to enemy eaves droppers listening in on radio transmissions and field telephone conversations.
Below are some examples of Sprechtafel forms. The last one includes English translations and annotations.
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WW2 German Field Telephone Equipment 