Because it _is_.
Because I know how such things work.
Because you -- the eavesdropper -- cannot tell which parts of the 'hiss' belong to which end of the conversation.
Because *BOTH* ends use the _same_ frequencies. At the *same* time.
At either end, by "knowing" what _you_ are putting on the wire, you can 'deduce' what the other end is doing, by 'cancelling' your signal from what you 'hear' on the wire.
WITHOUT that 'insider' knowledge, your best bet is "Miss Cleo".
This is also why there is all that hissing going back and forth _before_ the modems 'connect' -- they are each 'seeing' what the line conditions look like, *without* 'near-end' interference, when the far-end modem does 'known things' on the line. *Then* they both start talking, still saying 'known things', to verify that they can filter out their own noise and recover what the far end is saying.
*ONLY*THEN* do they announce 'connect', and let "user" data flow across the connection.With _extremely_high-end_ equipment that has been precision calibrated and the artifacts of gear identified and compensated for, *and* with a non- trivial amount of testing with known data on the suspect phone circuit, one _may_ be able to decode the bit-stream.
It is far simpler to play "man in the middle" by using a pair of your own modems, and intercepting the call start-up. one end (a) talks to your 1st modem, "thinking" it is talking to the far end (b), wile your
2nd modem is talking to that far end, in it's place. Now, you've got the recovered bit-stream passing between your 1st and 2nd modems, in decoded digital form, with the two halves of the conversation separated. reading _that_ is 'trivial', as they say.of course, going _that_ way means that you run afoul of any number of laws forbidding 'wiretapping', and/or interception of wire communications, but you're not going to let a little thing like _that_ stop you, are you? After all, you have the same legal issues with taking a recording of the modem 'conversation' in the first place.