Matthew writes to tell me about Max Neuhaus and his remarkable Radio Net Project from 1977: "Max Neuhaus, who is a musician (percussionist), inventor, something of a visionary. He’d figured out, given the way Public Radio was engineered in those days, that he could take the phone line networks and hook them together ... to create a piece of music by inviting listeners to phone in to their local public radio station and whistle into the phone. The sound would then go round the phone line network, run thru a phase shifter, and in a matter of seconds come out on the radio in a rather mysterious, layered texture of sound." Matthew's father made a documentary half hour documentary on Radio Net, which you can now watch here (Click on "A look behind the scenes at Radio Net's realization process"). There's a nice description of Radio Net, with diagrams here.