| I've yet to read a good explanation of why the telcos permit CLID faking and reinjection of apparently local CLID by overseas inputs. I'm assuming there's a technical and/or willpower reason or some counterfactual like VOIP depends on it. Even just flagging it would help. Or, rejecting numbers they can know lie inside their own routing architecture, or asserts within their own number plan where the CLID does not match. Morally it's like BCP38 in the customer facing internet systems: reject customer input they don't pay you to assert. |
The historic reason was, just like the Internet, the international phone network was built on gentlemen agreements by engineers who largely trusted each other.
A big national telco is unlikely to attack its peers, so there was little need for safety measures. As smaller telcos came in to the mix via deregulation, that understanding changed - but it was hard to retroactively fit controls.
The more modern reason is outsourced call centres. You want outbound calls from your Philippines based staff to show as if they were calling from a local number. When large and reputable entities were doing this it was fine. Just like showing a different reply-to address on an email.
If you were designing a modern network, it wouldn't be like this. But international telephony is over a hundred years old and has a huge amount of legacy technology and legal agreements.