> The phone system largely does not track or care about the actual source of a phone call
Sure they do. When I worked for a small telecom, the provenance of every call was meticulously tracked for billing purposes. Figuring out who to bill for what on each link of the chain is tricky and requires excellent recordkeeping.
That system is entirely unrelated to Caller ID, though.
It's also unrelated to who's actually calling - only the originating carrier. In the case of VoIP gateways (where basically all of this traffic ultimately originates), billing ANI typically gives you one number for the entire carrier that handed over the traffic. Since these calls often traverse multiple carriers (across multiple countries often) that's usually a billing indicator for a carrier that's already one or two steps into the PSTN.
ANI does tell you something, but it is not the number of the caller, it's an identifying number their carrier has chosen to use for billing purposes! For most calls not originating from POTS that's basically an arbitrary identifier and normally different from CID and other properties of the caller. Even on a lot of POTS calls it's different, for example, with institutional customers using PABXs where there is usually only one ANI number regardless of the number of DIDs etc they possess.
Sure they do. When I worked for a small telecom, the provenance of every call was meticulously tracked for billing purposes. Figuring out who to bill for what on each link of the chain is tricky and requires excellent recordkeeping.
That system is entirely unrelated to Caller ID, though.