One word of caution when using 'buzzer' signalling phones with 'magneto' signalling ones. I have tried a Model D (buzzer ringing) with a Model F (magneto ringing) and they work well, except when the buzzer key is pressed on the Model D
during a call. The buzzer note that emanates from the receiver of the Model F is deafeningly loud
...a feature that so amused a group of Scouts recently that I had to revert to a pair of Model Fs on Health and Safety grounds, to prevent hearing damage!
Does this happen when using a pair of Model Ds, or is mine faulty? These are equipped with both buzzer and bell so clearly they were designed to be inter-operable with other types of ringing system. Perhaps the buzzer was supposed to be disabled off-hook, but I've read somewhere that the miniature Morse key was designed for sending messages by Morse Code when battlefield noise was too great for voice traffic to be heard.