A reed relay used for detection of hanging up the phone handset
The image was copied from Wikimedia Commons (accessed 2023-11-11): https://commons.wikimedia.org/wiki/File:Auerswald_COMfort_2000_Base_-_Handset_-_reed_switch-2363.jpg
phone_handset_reed_switch_by_raimond_spekking.jpg
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Raimond_Spekking, CC-BY-SA-4.0