Ringer 45 - British Post Office motor driven telephone ring generators starting up and shutting down by ElmerCat published on 2013/04/03 16:12:25 +0000 Remarks by British telephone historian, Paul Coxwell: " It's British Post Office. As somebody is selecting the various tone outputs to monitor at the beginning, you can hear, in sequence, continuous ringing output, N.U. (Number Unobtainable) tone, dial tone, E.E.T. (equipment engaged tone - the alternate long & short), then busy tone. There's a recording with video clip of the interrupters of a different BPO ringing machine here: http://www.youtube.com/watch?v=VzIXvO6RahQ -Paul C. " The discussion thread on that YouTube video has more information about this long-lost sound producing technology. Back then, millions of people heard the music of mechanical tone generators every day!