[ralferoo]

I'm from the UK, but IIRC I think the US phone numbers originally used the 2nd digit to discriminate whether the first 3 digits were a local (7 digits) or national (10 digits).

Not sure where your 5 digit example fits into that though, that'd suggest there was actually 3 levels of discriminators originally. It's certainly plausible when there were relatively few numbers used, with the push to 7 and 10 digits only when the number space was getting fuller so that those prefixes could be reused.