They weren't using the POTS line for power (though I remember holding the leads when a ring signal came in - quite a shock! - seemed like the NYC local-loops must have had a RI# of about 20 at the time...) -- and they weren't just building a speaker - they were building a speaker in the same enclosure as microphones, so physical isolation seems like it would be top on the list. (Though even that would have only helped with freqs <= a few hundred Hz, so their active cancellation would still be key...) No big deal - it just "seems" obvious, but I wasn't there...