When I was growing up we didn't use the area code, so the phone number I still have burned into my brain despite it not having been active for nearly 20 years was 8 digits, beginning with 3. Area codes, if we had used them, would have added two digits, the first of which is always 0 in Australia.

That's landlines. Mobile phones were only just starting to become popular when I first moved overseas as a kid. They're always 10 digits, and always start with 04. In both landlines and mobiles, you drop the leading 0 if you're going to add the country code.

In Korea, Vietnam, Malaysia, and Austria, mobile phone numbers without a country code have a leading zero. If there's somewhere that doesn't do this, I suspect they are an outlier.

In the 90s if you share the same area code, you don't need to enter it. This started to be a problem when phone numbers started to run out especially in metro areas. I remember putting in the area code because you needed the area code to dial in to ISPs when I was using a 56k modem.

My family got a brand new Pentium 3 computer that came with Windows ME. Part of the install/setup/onboarding process of this OS was connecting to the internet via its dialup modem. My father's work was our ISP, it was a local number. We left the area code field blank, put in the 7 digit phone number, and the software wouldn't accept that. The software required the area code field to be filled in. We filled it in, and it pumped the modem noises through to the speakers, where we heard "doo Daa DEEE It is not necessary to dial the area code for a local call. Please hang up and dial the phone number without the area code."

ENGINEERING!