Overlays and splits: why your city has more than one area code
When a region runs low on phone numbers, the system adds a new area code one of two ways — a split or an overlay. Which one is used decides whether your number changes and whether you dial all 10 digits.
Split vs overlay
| Split | Overlay (modern) | |
|---|---|---|
| How the new code is added | Region divided geographically | New code laid over the same area |
| Does your number change? | Yes — for one side of the split | No — nobody's number changes |
| Dialing | 7 digits locally (historically) | 10 digits mandatory for everyone |
| Used today? | Rare | Almost always |
Why you now dial 10 digits
With one area code, 7 digits was enough. Once an overlay adds a second code over the same area, 7 digits is ambiguous, so 10-digit dialing becomes mandatory — even for local calls.
See cities served by multiple codes · By state & province
Frequently asked questions
Why does my city have more than one area code?
Because it ran out of available phone numbers in the original code. Rather than change everyone's number (a 'split'), the phone system now usually adds an 'overlay' — a second area code covering the exact same region — so new numbers can keep being assigned. That's why one metro can be served by three or four area codes.
What's the difference between an overlay and a split?
A split divides a region into pieces and gives one piece a brand-new area code, so people in that piece have to change their number. An overlay lays a new area code over the same region without changing any existing numbers — the trade-off is that everyone must dial all 10 digits. Modern capacity relief is almost always done with overlays.
Why do I have to dial 10 digits now?
Because your area has an overlay. When two area codes cover the same region, the phone network can't tell which one you mean from 7 digits alone, so 10-digit dialing (area code + number) becomes mandatory for everyone in the overlay — even for local calls.
Does an overlay change my phone number?
No. Existing numbers keep their original area code. Only newly assigned numbers get the new overlay code. (A split, the older method, did force number changes — which is part of why overlays replaced it.)
Sourced from the official NANPA (North American Numbering Plan Administrator) numbering database, current as of August 2, 2026. Refreshed monthly.