I posted the photo of the back side of the receiver (earpiece) for a Bell System telephone (either 500 or 2500) set. I said that the green blob in my photo (and the ’44A’ metal can in the wikimedia photo below) was used for reducing the volume of loud signals. Well, that’s only slightly correct. Another commenter said that it was called a click suppressor – see the following facebook link; he’s right.
https://m.facebook.com/groups/ElectronicParts/permalink/2280534545469201/?comment_id=2280587508797238
https://commons.m.wikimedia.org/wiki/File:Western_Electric_Type_44A_varistor_on_U1_receiver.jpg#mw-jump-to-license
Someone else gratiously put the following telephone wiring diagram in a comment – it shows the green blob as two diodes antiparallel across the receiver element in the upper right corner. When the voltage at the receiver goes above a certain level, these two diodes conduct and limit the loudness, such as when a loud click is received.
not from the Bell System (at&t)
Back to what I said about the telephone set compensating for weak signals to be louder and the strong signals to be not so loud. The telephone still does that, but not with the green blob on the receiver. Instead, it has the two devices marked with 1 and 2 double crossed ‘%’ signs, which are described at the bottom of the notes as ‘voltage dependent resistor’, black dot and yellow dot.
This wiring diagram is not from the Bell System AKA at&t. The Bell 500 and 2500 sets have a network with all of the components inside, and it’s sealed in viscous silicone snot – if you try to get it out to see what’s inside, you’ll be covered by the icky, sticky clear silicone snot! So it’s likely that aanyone who has worked on the Bell telephones has ever seen inside of the network.
But the network has those same VDRs – voltage dependent resistors, which compensate for weak or loud signals. If there is a long distance between the central office and the telephone, the current through the telephone line will be lower and the VDRs will use less current, leaving more for the telephone, so the signals will be stronger. If the telephone is close to the central office, the VDRs will use more of the current, and the signals will not be too loud.