[matthiasl]

The discussion is new to me.

I started analysing the audio because someone sent me a link to the film (The Wall) on youtube and asked me about the signalling. Once I'd decoded the telephone number, I tried googling it, to see if someone else had already figured out what it was (a US local number? the number to a US operator? the number a US operator called to talk to a UK operator? the number a UK operator dialled to get a London number?), but nothing came up. There's quite a bit of good discussion about that in the comments here.

A week or two later, I tried googling 'Pink Floyd Telephone Call', and found that the audio actually comes from the album, i.e. it's not just in the film, and a bit more information about how it was made, and put that in the addendum.