The three parts of a US phone number
Every standard North American number breaks into an area code, an exchange code, and a subscriber number. Telecom engineers call this the NPA-NXX-XXXX format: NPA (Numbering Plan Area) is the area code, NXX is the three-digit exchange that used to map to a specific switching office, and XXXX is the four-digit line number. New Jersey's own numbering history shows a fix that's rarer than the split-versus-overlay choice covered above: three separate area codes went live there on the exact same day, a coordinated launch most states never needed.
212 wasn't random: the original 1947 codes were built around rotary-dial speed
When AT&T rolled out the first 86 area codes in 1947, it assigned them using a rule tied to rotary-dial phones, not geography: cities expected to receive the most long-distance calls got the area codes that took the fewest rotary "pulls" to dial, since each digit's dial pull took time roughly proportional to its value. New York City, the most-called destination in the country at the time, got 212, as low a pull count as a 3-digit code with a required middle digit of 1 could get. Los Angeles got 213, and Chicago got 312, for the same reason.
There was a second rule behind the middle digit specifically: any state or province with more than one area code got a code with 1 as its middle digit (212, 213, 312...), while states with only a single area code got a 0 in the middle instead (202 for Washington D.C., 401 for all of Rhode Island). That's a fossil of the 1947 plan you can still spot in a lot of today's area codes, even though the underlying rotary-dial logic stopped mattering decades ago.
Split vs. overlay: two different fixes for the same problem
An area code eventually runs out of available NXX exchange blocks as an area's population and phone/device count grow. Historically, the fix was a split: draw a line through the region, keep the old code on one side, and assign a brand-new code to the other. Los Angeles's original 213 was split this way five separate times between 1951 and 1998, each time handing off a geographic slice to a new code (714, 805, 818, 310, 323).
Splits have a real cost: everyone on the losing side of the line has to change their number's area code, and businesses have to reprint everything. Starting in the 1990s, the industry mostly switched to overlays: rather than dividing the region, a second area code is layered on top of the exact same geographic footprint, and only new phone numbers get assigned to it. Existing numbers never change, but everyone in an overlay area has to dial all 10 digits (area code included) for every call, even a next-door neighbor. Manhattan is the extreme example: 212 (1947) was overlaid by 646 (1999) and then 332 (2017), both layered on that same Manhattan-only footprint, plus 917 (1992), a citywide overlay that covers all five boroughs and reaches Manhattan too, so a Manhattan phone number today can carry any of four different area codes, even though only three of them (212, 646, 332) are Manhattan-exclusive.
An area code doesn't reliably tell you a timezone anymore
Because area codes map to numbering plan areas that were drawn decades ago and don't get redrawn when state or timezone boundaries shift in practice, and because mobile numbers travel with their owner instead of staying tied to a location, a given area code is a much weaker timezone signal than it looks. A 915 number was assigned in El Paso, Texas, which observes Mountain Time even though the rest of Texas is Central, and a person who got that number in El Paso in 2015 keeps dialing 915 no matter where in the country they've moved since.