[deadcore]

The how - depending on the protocol.

Signalling System No. 7 - ISDN User Part spec (found here: https://www.itu.int/rec/T-REC-Q.763-199912-I/en) allows you to specify both a calling party number (3.10) and generic number (3.26) (the UK spec adds an additional presentation number so you have 3). This will typically require the help of an operator which is 'connected' to the network on the PSTN. A real business case can be made; like a generic, non geo support numbers appearing on the persons phone instead of the geographical number of the office which called. Either a bit of social engineering or findings a less scrupulous operator is all you really need to do

SIP has FROM and P-Asserted-Identity headers which follow the same process

[fxtentacle]

Thank you for this rabbit hole. Today I learned a lot about modulation, frequencies, and how DSL works :)

In the end, the most surprising snippet of knowledge for me was that Erlang (that Amazon S3 is built in) was invented by Ericsson for live patching ISDN phone routing systems without dropping any ongoing call.

[FigmentEngine]

I do not believe S3 is written in Erlang... do you have a source for this?

[drcongo]

Does that addition in the UK spec add any extra protection? I've never had a spam call with a spoofed number in the UK as far as I'm aware, and definitely never seen that thing that happens in the states where a call comes in showing the name or company name of the caller, even if they're not in the receiver's address book.

[deadcore]

The UK spec adds a few fields which I can not remember all off the top of my head. One addition is the 'presentation' calling line identifier, which is screened like the 'calling party number'. The generic calling line identifier is not screened, hence the addition

ref what the screening bits are: https://www.dialogic.com/webhelp/csp1010/8.4.1_ipn3/exsapi_q...

As for if the UK & spoofing, it's a very real thing with very real business cases and abuses.