There's an entire process / plan in place that adds new area code overlays when the existing one is getting close to being exhausted. 618 is overlaid with 730 and will be exhausted probably in 2025 or 2026.
New area code 557 to overlay on 314.
There's almost 8M numbers available in an area code and a little over 1M people in 314. We really ran out?
Most are consumed by businesses and corporate entities, though the proliferation of the cell phone means a family now has 2-5 numbers instead of a single landline. When you think about business entities that may have various departments, any number of direct call in lines to specific management or sales personnel, etc., given our 1M population in the 314 I'm actually impressed that it wasn't exhausted well before this point in time.
To your question why 314 is running out of numbers - IP phones and businesses. Phone numbers today are far from just a 'one number per person' thing.
I believe they are considering numbers not just currently in use but blocks reserved for specific businesses and/or mobile providers. IP Telephony providers reserve huge swaths of numbers for future use. And business Robo-dialers - for both legitimate and illegitimate purposes - bank tons of numbers too.
Companies who use call centers also often block out large numbers of outgoing numbers and their robo-dialers rotate through them randomly- so you can't avoid them by blocking a single number. So a single employee working for, say, a collections company, may have one internal phone number for company use, another direct line for inbound customer calls, and when he makes an outbound call the phone system will call a robo caller which will cycle through a couple hundred different numbers.
And for phone providers, take Google Voice, for instance. They have a bank of 314 and 636 area code numbers reserved for use - some active, but a ton just reserved and sitting idle so if I want to go download Google Voice and set up a new account I have a list of 'available' 314 area code phone numbers (from Google's pre-approved pools of reserved numbers) I can pick from. And there are loads of similar examples - from Google Voice competitors to VOIP phone providers for residential or businesses.
In addition, specific prefixes are assigned to specific mobile carriers. 314-503-xxxx and 314-495-xxxx, for example, were specifically allocated to Sprint back in the day (now T-Mobile). So if you see a number with that area code and prefix you can be relatively certain that's a mobile number (unless it's being spoofed, which is a thing). But that means AT&T can't just assign a new 314-503 number to an office phone downtown, even if there are a bunch of 314-503 numbers left unused today.
So all of that means that big chunks of 'available' numbers aren't as available as you might think at first glance.
I don't know if many remember, but they were going to overlay 557 about twenty years ago due to what RBB was saying in the last paragraphs. New providers (Cingular, Ameritech) moving into the market were gobbling up phone numbers for potential customers. It was the Wild West, cellphone providers were popping up out of the woodwork buying up numbers. Eventually attrition and consolidation released a lot of numbers.
