Note the 2 uf capacitor in series with the bell. This is important when dealing with AC superimposed on DC. A capacitor will block DC current but will allow AC to pass.Because the rest of the system never see's it.
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As the diagram shows your phone is normal open till one lifts the hand set.
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In this case, the capacitor will cause the bell to ring when AC is applied, but it won't allow the DC to burn up the coil.
