Possibly, but I'd wager more that it was due to Bell owning the entire network, so they knew that any "from" numbers were correct. If I'm reading Wikipedia righ

Possibly, but I'd wager more that it was due to Bell owning the entire network, so they knew that any "from" numbers were correct. If I'm reading Wikipedia right, SS7, which is still supported today, was created all the way back in 1975![0] Bell wasn't broken up until 1982.[1] [0]: https://en.wikipedia.org/wiki/Signalling_System_No._7 https://en.wikipedia.org/wiki/Signalling_System_No._7 [1]: https://en.wikipedia.org/wiki/Breakup_of_the_Bell_System https://en.wikipedia.org/wiki/Breakup_of_the_Bell_System