Okay, so the PSTN has two things in it, ANI and Caller ID - and they're not related, the ANI will reflect the actual account that the call comes from (Often its

Okay, so the PSTN has two things in it, ANI and Caller ID - and they're not related, the ANI will reflect the actual account that the call comes from (Often its passing the BTN (billing telephone number)), and the Caller ID is either a Telco set, or user set number string (ISDN NI-1 for example you can't send a CID string, NI-2 you can, SIP you also can). Now, most calls, the BTN/ANI and CID are the same number string, but when you start looking at call center operations, calls from PBX's, anything with backup trunking arrangements - that gets less and less true. Another wrinkle in this whole mess, is that with LNP (local number portability) you can have a number that on paper is owned by carrier Y (by assignment) but has actually been ported to carrier X (there is a database however that carries this information). The issue as it comes down, is there is no central database to show that Caller ID string X is valid for trunk/line Y, for the most part customers were trusted to not be malicious and follow the rules. Once we got into a world where a SIP trunk could be had for free, and usage was a minor incremental charge, that whole system broke down. The good news in all of this is that the FCC has mandated a move to authenticated CID (https://www.fcc.gov/call-authentication https://www.fcc.gov/call-authentication), but much of the equipment in play here is decades old, and will take time to be upgraded - but it's happening and happening pretty quickly for an industry that normally moves at a near glacial pace.