Earlier quoted context omitted.
I'm surprised "Four Oh [X]" is specifically a Canadianism. I would have expected it to manifest more broadly, given things like: https://en.wikipedia.org/wiki/409_(song)
The Atlanta area code is definitely "four-oh-four" (and also the HTTP error)
I don't think I don't ever use "oh" for zero. FWIW, I'm from South Florida with New Jersey parents. The area code I grew up with is "three-oh-five".