This sounds like it was one of those "princess" style trim-line phones with
all the electronics in the handset. In these cases the filter must be as
close to the handset as possible since that's where the audio rectification
is happening. I used K-comm filters on these phones with good results
since they hang off of the handset but are pretty light so they don't reck
the plug or jack with the weight of a ferrite.
73, de ed -K0iL
But even THAT may not prove to be the final word --- my neighbour three
houses away years ago had RFI from my 10-meter transmitter with anything
over 25-watts output, but ONLY WHEN SHE WAS STANDING UP!
Turns out the handset cord was acting like a helically-wound, resonant
vertical when outstretched like that --- when she sat down by the phone
table (& the cable was bunched-up & short) there was no RFI...and yes, I did
have the Rich Measures' filters/toroids on both the handset cord, AND the
mainline, as they entered the telephone...
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