How area code overlays work

What an overlay changes

In an overlay, a new area code is added within the same geographic area served by an existing code. Existing customers keep their numbers; new numbers can use the newer code.1 The Canadian administrator describes the same mechanism for Canadian numbering areas, and notes that numbers from the new code are assigned to new growth on a first-come, first-served basis.4

For example, NANPA's current database lists 202 and 771 together as the in-service overlay complex for the District of Columbia, with 202 dated 1947 and 771 dated 9 Nov 2021.3 The codes are different, but the overlay area is the same. An older-looking code is not inherently more local.

Splits, realignments and overlays

An overlay is one of several ways to relieve an area code that is running out of prefixes. Federal rules name three: a geographic split, in which the area is divided and one part gets a new code; a boundary realignment, in which codes with spare capacity are moved across the border between two adjacent codes; and an all-services overlay, which adds a code to the same area.5 Each has a cost to customers. A split changes the area code for roughly half of the customers, so stationery and advertising must be revised, but it keeps seven-digit dialing within each code. An overlay changes no one's number but requires ten digits for every call in the area.1

State commissions decide which method to use, along with the new boundaries and the dates.5 The overlay has become the usual choice: NANPA notes that it avoids the public and political argument over split boundaries, and that subsequent relief will likely be another overlay.1 The Canadian administrator lists the variants used in Canada, from distributed and concentrated-growth overlays to a multiple overlay that covers several codes with one new code.4

The all-services rule

Overlays must be open to every kind of service. Federal rules say that no all-services overlay may be implemented unless the numbers in the new code are assigned first-come, first-served regardless of the identity of the carrier, the technology or the type of service, except where the FCC authorises a technology-specific overlay.5 NANPA explains why: a 1995 FCC order essentially banned setting aside a code for wireless or fax, because callers to that code would need ten digits while wireline-to-wireline calls used seven, favouring wireline customers.1 This is also why the area code of a business number does not reveal whether the line is wireless, wireline or VoIP.

Why ten digits matter

When two codes serve the same area, the seven digits after the area code can repeat. The area code therefore becomes part of a local call. NANPA says US overlay areas require ten digits, or 1 plus ten where the state requires it, as an FCC requirement for dialing parity between the codes; Canadian guidance also calls for ten-digit local dialling with overlays.14

NANPA describes a three-step introduction: a permissive period in which seven-digit calls still complete while customers reprogram equipment; a mandatory date after which seven-digit calls are sent to a recording that stays indefinitely; and the introduction of numbers in the new code at or shortly after that date.1 Three-digit services such as 911 and 988 are unaffected.1 Once a relief plan is approved, NANPA publishes a planning letter summarising the plan and its implementation schedule, and press releases announce the dialing-plan change.2 Business customers are told to update PBXs, fax machines, alarm dialers and call-forwarding entries, and to ask for a test number, which NANPA says is established at least 30 days before permissive dialing starts.1

Show the complete number on your website, printed materials and saved contacts. If a team member has an old seven-digit shortcut, update it when the local dialling plan changes. The same applies to the mobile numbers your call routing forwards to.

Reading real overlay groups

The widest in-service overlay complexes in the file are 236/250/257/604/672/778 (British Columbia), 368/403/587/780/825 (Alberta), 236/250/257/672/778 (British Columbia) and 236/257/604/672/778 (British Columbia).3 Each group is one place served by several codes: a caller in that place can be reached on any of them, and a business in that place can be assigned a number from whichever code has numbers free. In Canada the position is the same; the regional pages mark where the administrator's coverage labels differ.4

Of the 145 in-service complexes recorded across the regions in this reference, 112 have exactly two codes. A third or fourth code usually means the area has needed relief more than once, which each region page shows in its history section.

How to read the regional tables

The region pages list only geographic codes already in service. Each table shows the in-service peers named in NANPA's overlay-complex field.3 Some source complexes also mention future codes; those are omitted from the visible peer list and described in each page's section on pending codes. A province or state can contain several distinct coverage areas, so being listed on the same regional page is not proof that two codes overlay each other.

Choose the locality first, then ask which numbers can be supplied there. If your existing number already works for customers, porting it may be simpler than introducing a new code; the overlay and 10-digit dialling glossary entries give the short definitions. For a non-geographic alternative, see the toll-free code list, and for a worked example of choosing among codes read the area-code guide.