The calling needle was similar to the signal needles with some additional mechanical apparatus. The needle was suspended by metal wire, rather than a silk thread, and had attached to it a horizontal arm. When a call was made the arm turned and pushed over a lever with a lead weight attached which fell under gravity and released the detent of a clockwork alarm. In specifying a separate calling line and mechanism, Schilling was following the arrangements on the electrochemical telegraph of Samuel Thomas von Sömmerring.Fahie, p. 312 Schilling had become familiar with Sömmerring's work while a diplomat in Munich and frequently visited Sömmerring to see and assist with his telegraph.Huurdeman, p. 54
