How US Area Codes Change: Overlays, Splits and 10-Digit Dialing
Why your region suddenly gets a second area code, who decides it, and what actually changes when 10-digit local dialing becomes mandatory.
A letter from your phone company says a new area code is coming. Your first question is probably whether you'll have to change your number. With an overlay, the answer is no: current customers keep both their area code and their number. What changes is how you dial. Once two codes share the same territory, every call needs the area code.
Below: why codes run out, who signs off on a new one, and the devices that tend to break on the mandatory date.
Why an area code runs out of numbers
An area code isn't a bottomless pool. It holds 792 usable prefixes (the three digits after the area code), and each prefix holds 10,000 numbers.
Why 792 and not 1,000? Prefixes can't start with 0 or 1, which knocks out 200 combinations. The N11 codes, like 911, take away 8 more. So in a number such as 313-555-0142, the "555" slot can never be 055, 155 or 411.
Every prefix handed out to a phone company comes from that fixed pool. When the unassigned ones run low, planners call it "exhaust," and the area needs relief, which in practice means a new code.
Who decides when and where a new code goes
The North American Numbering Plan Administrator (NANPA) watches the supply. It publishes exhaust forecasts for every area code twice a year, in April and October. When a code is expected to run dry within 36 months, NANPA alerts the state regulator and the phone industry.
NANPA doesn't make the final call, though. The state commission or regulatory authority decides on every area code relief plan. That's why the timing, the dialing rules and the public notices look a little different from one state to the next.
If you're wondering which codes already serve your state, our list of US area codes by state shows them together. The North American Numbering Plan page covers the wider system that these codes belong to.
Area code split vs. overlay
There are two main ways to add a code, and they hit people very differently.
| Method | Existing numbers | New numbers |
|---|---|---|
| Geographic split | The territory is divided and roughly half of customers get a new area code. | Each side of the new boundary uses its own code. |
| Overlay | Everyone keeps their area code and number. | New customers or added lines may get the new code. |
With a split, a dentist on the wrong side of the new line has to reprint every appointment card and update every listing. With an overlay, nobody's existing number moves. The catch is that two neighbors on the same street can end up with different area codes, and everyone has to dial all ten digits.
Why local calls now need the area code
Picture an overlay like Indiana's, where 317 and 463 cover the same ground. A number like 555-0123 could exist under both codes. Dial only seven digits, and the network has no way to know which one you meant. So once two codes share an area, every call has to include the area code, and some states go further and require 1 plus the 10 digits.
There's also a rule behind it. The FCC made 10-digit dialing a requirement for overlays in its Second Report and Order (FCC 96-333). The goal was dialing parity between the old and new codes, so established carriers holding the old numbers wouldn't get an edge over newer ones.
After mandatory dialing starts, a seven-digit call reaches a recording that tells you to add the area code. That recording stays in place indefinitely.
One worry you can drop: an overlay doesn't make your calls pricier. Asked whether a new area code changes what calls cost, NANPA's answer is "No." Calls that were local stay local, and local calling areas don't move.
How the switch rolls out
An overlay doesn't flip overnight. During the permissive 10-digit dialing period, you're encouraged to start dialing the area code, but seven-digit calls still go through. Treat it as practice time. If you wait until the mandatory date to discover that your alarm panel dials seven digits, the recording will be the first sign.
NANPA's list of things to reprogram during that window is specific: alarms and safety systems, office PBX phone systems, fax calling lists, speed dialers, gate and door entry systems, auto-dialers and dialing lists on computers.
Two real overlays: Indiana and Michigan
Indiana's 317 area went through this a decade ago. Ten-digit local dialing became mandatory in area code 317 on October 15, 2016, under an Indiana Utility Regulatory Commission order. From November 15, 2016, new numbers in the region could come from either 317 or the new 463 overlay. See our page on Indiana area codes for the state's full set.
Detroit is the recent case. The Michigan Public Service Commission approved 679 as an overlay on 313, which serves Detroit and nearby Wayne County suburbs. Since October 7, 2025, local calls inside 313 must use 10 digits, and seven-digit calls no longer complete. The Michigan area codes page lists the rest of the state's codes.
Don't assume either state's dates apply where you live. If you're in California or anywhere else, check your own state's notices.
10-digit dialing without a new area code: the 988 change
Maybe your area never got a second code, yet you still had to start dialing ten digits. If so, 988 is probably why. On July 16, 2020, the FCC adopted an order (FCC 20-100) making 988 the three-digit code for the national suicide prevention and mental health crisis hotline.
The trouble was that in some area codes, 988 was also an ordinary local prefix. Where seven-digit dialing was still allowed, a local call to a 988-xxxx number and a call to the hotline began with the same three digits. The FCC's 988 order required carriers to make 10-digit dialing mandatory in area codes that still allowed seven-digit local calls and also used 988 as a local prefix. NANPA's transition plan set October 24, 2021 as the mandatory date in those codes.
Three-digit service codes still work either way. You can dial 911 and 988 with three digits after a new area code arrives, and the same goes for 211 through 811 where they're offered.
A quick checklist before the mandatory date
- Open your phone's contacts and add the area code to any entry saved with seven digits.
- Call your alarm or medical-alert company and ask whether the panel dials out with seven digits.
- Check the gate or building entry system, fax machine and office phone system.
- Look at business cards, your website, invoices and signs that show a seven-digit local number.
- Teach kids the full ten-digit home number, not just the last seven.
One last habit worth keeping: an area code you don't recognize isn't automatically local. If a call from one rings once and hangs up, read our guide on unfamiliar +1 area codes before you call back.
FAQ
What is an area code overlay?
It's a new area code added on top of the territory of an existing one. Current customers keep their numbers, while new customers and added lines may get the new code.
Why do I have to dial the area code for local calls?
Because two codes serve the same area, the same seven digits could belong to two different numbers. Every call must include the area code, and some states require 1 plus 10 digits.
Will my phone number change when a new area code is added?
Not with an overlay: existing customers keep their area code and number. With a geographic split, roughly half of customers have to change area codes.
Will local calls cost more after an overlay?
No. Calls that were local before the new code stay local.
Can I still dial 911 or 988 with three digits?
Yes. Both still work as three-digit numbers after a new area code arrives.
Who decides where a new area code goes?
NANPA forecasts exhaust and warns regulators, but the state commission or regulatory authority makes the final decision on each relief plan.
Sources
- North American Numbering Plan Administrator (NANPA): Area Code Relief Frequently Asked Questions (October 28, 2024) (accessed 2026-09-24)
- North American Numbering Plan Administrator (NANPA): NPA Relief Planning (accessed 2026-09-24)
- Indiana Office of Utility Consumer Counselor: 317 Area Code: 10-Digit Dialing Now Required for Local Calls (accessed 2026-09-24)
- Michigan Public Service Commission (press release hosted by NANPA): 10-Digit Dialing Required in the Michigan 313 Area Code Starting October 7, 2025 (accessed 2026-09-24)
- North American Numbering Plan Administrator (NANPA): Planning Letter PL-556: Transition to 10-digit dialing (for 988 as 3-digit access to National Suicide Prevention Lifeline) (accessed 2026-09-24)