The phone system appeared in other songs of theirs. The intro to ‘Money’ from Dark Side of the Moon had the cash register/coins/drawer shut followed by the old Strowger switch – the mechanical stutter sound. I still hear this sound in my head from my exchange maintenance days – there would be a cacophony of sound as you walked around the exchange – H&S would have had a field day, forcing engineers to walk around with ear protectors!! Unfortunately you had to listen out for alarms when switches stuck or incoming calls from other exchanges for co-op clearing a problem.
I must confess I always thought the tones were just Blue Box Phreaking noises!
The tone calling systems were like an earlier version of the old fax machine’s handshake, just separated to enable the switches to connect ( and clear ) forward to the ultimate destination, the person called. In fact an engineer could piggyback onto a call and listen and recognise the tones as the call was successfully set up and connected.
The Americans used an outdated system that let ‘phone phreakers’ set up a free call, then use the tone machine to try to clear it down and set up a ( chargeable ) long distance call ( Blue Box Phreaking. Steve Jobs/Wozniak made some of these Blue Boxes as an earlier illegal side-line prior to starting Apple.
The tone system used by the UK ( and most parts of Europe ) was 2280Hz – this was researched when the switches were developed and found to be the frequency least used by the human voice in conversation, but still within the range that the old telephone system was optimised for – 200Hz to 2400Hz.
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