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Glossary · Numbering

What is Area code overlay?

  • Also called: overlay code, overlay area code

Reviewed by Sujan ThapaliyaLast updated

Definition

An overlay is a new area code introduced across the same geography as an existing one, rather than splitting the region in two. Nobody has to change their number, but everyone in the area has to dial ten digits, because seven no longer identifies a number uniquely.
The three STIR/SHAKEN attestation levels, which underpin how area code overlay behaves in practice: A asserts both that the carrier knows the customer and that the customer may use the calling number, B asserts the customer only, and C asserts neither. A shared number pool can reach B at best, which is why number ownership keeps recurring in these definitions.
Attestation is the backdrop to most caller-ID and deliverability terms, area code overlay included.

In practice

How area code overlay actually works

Overlays replaced splits because splits forced half a region to reprint everything. The trade is that local dialing changes for everyone instead of the number changing for some.

The visible consequence is a city with two or more area codes and no geographic distinction between them. New numbers get the new code, which is why an unfamiliar code can be genuinely local.

For anyone running local presence, overlays mean holding numbers in both codes for a metro, or accepting that a share of recipients will not recognise the newer one.

Overlays also change what a local number signals. In an overlay area a business with the original code reads as established and one with the new code reads as recent, whether or not either is true — which is why some businesses pay a premium in the secondary market for a number in the older code.

Worked example

Before an overlay, seven digits reached your neighbour. After it, the same seven digits are ambiguous between two codes, so every local call needs ten — and nobody's number changed.

Area code overlay: common questions

Why do I have to dial ten digits for a local call?
Because an overlay code shares your area. With two codes covering the same geography, seven digits no longer identify a unique number.
Does an overlay change my existing number?
No. That is the point of an overlay rather than a split — existing numbers keep their code, and only new assignments use the new one.
Is a new overlay code less trusted by recipients?
Initially, yes. An unfamiliar code in a familiar city is answered less often until people get used to it, which is worth knowing when choosing numbers for outbound calling.

Sources

  1. Combating Spoofed Robocalls with Caller ID Authentication — Federal Communications CommissionThe STIR/SHAKEN framework, the attestation levels carriers sign calls with, and the mandate requiring providers to authenticate caller ID.
  2. 47 U.S.C. § 227 — Restrictions on the use of telephone equipment — Cornell Legal Information InstituteThe Telephone Consumer Protection Act itself — the consent requirements, calling-hours limits, and private right of action.
  3. ITU-T Recommendation E.164 — The international public telecommunication numbering plan — International Telecommunication UnionThe international number format, the 15-digit maximum, and how country codes and national numbers compose.

See area code overlay in the product

Local presence dialing presents a caller ID in the same area code as the person you are calling, because a familiar area code is answered more often than an unfamiliar one. Done properly it uses numbers you actually own and can receive callbacks on; done badly it is number spoofing with a marketing name.

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