This time I’ve got the adapter connected to both a 1988 BT Tribune analogue telephone set and a Tempo test set (the same ones that BT Openreach have. These test sets can monitor the line without seizing it (or anyone being aware you are doing so!)
Listen between the first and second ring; the noise that sounds like an old modem (it is indeed the same signalling) contains the caller ID info in ETSI format. A phone with CLID display (or other suitable equipment) can decode the incoming number from this and process it as required.
BTW the particular number I has already been shown online I’ve even put it up in full clear text on more than one site and its surprising how little spam I get to it – that does make me think the spammers have lists of what are higher value customers and only bother with those…
TBH if anyone actually manages to decode it from the FSK in the recording and send me a text I’d be quite impressed…
