I didn't see if and when this had been discussed before, so here goes:
Have you ever noticed that 90% of the time when the telephone rings on the Jack Benny radio show, the sound effect drowns out anything else that is going on at the time?
Now, I collect old telephones, and I have learned that to ring an old-style bell telephone, you turn the crank and it generates a large magnetic field and quite a respectable voltage.
My theory is that the sound effects man had to have the ringer close to the microphone to be able to be heard, but then the magnetic field from the telephone interfered with the mic.
And so the sound of the telephone isn't really overpowering the the actor's in the engineer's mix, but I think we're hearing the interference of the sound effects mic messing with the overall balance of the show. This is consistent with what I think I'm hearing.
Also, the 90% figure might be a little high, it might just stick out to me when it happens, and I don't notice it otherwise!
Any ideas? Feel free to shoot me down!