An area-code overlay is a new area code introduced to serve the same geographic area as one or more existing codes, instead of cutting the area in two. The US rules describe "an all services area code overlay, which occurs when a new area code is introduced to serve the same geographic area as one or more existing area code(s)", set it beside the geographic split, and allow one only where ten-digit dialling is mandatory "for every telephone call within and between all area codes in the geographic area covered by the overlay area code" (47 CFR 52.19(c)(1), (c)(3) and (c)(3)(ii), https://www.law.cornell.edu/cfr/text/47/52.19, accessed 27 September 2026).
How overlays show up in the data
NANPA's NPA report lists each North American area code with the overlay complex it belongs to and the code it came from, and Spaw syncs it daily. In the report of 26 September 2026, 628 sits in the 415/628 complex with 415 as its parent and has been in service since 21 March 2015, and 465, in service since 18 June 2026, is the newest code of the complex 718 shares with 347, 917 and 929. Codes in one complex share rate centres: NANPA's code file of the same date lists 483 codes of 415 and 76 of 628 in San Francisco's central rate centre. Whether a code was created by an overlay or a split is derived from the report, from whether its parent sits inside its complex.
What Spaw answers
The lookup has no overlay field, because an overlay is a fact about a whole area code rather than about one number. What an overlay changes is how far an area code can be read as a place: neighbours can hold numbers in different codes, and the area code alone no longer separates them. A number's region comes from its rate centre where the metadata names only the state, and the codes of an overlay share those, so a 628 number and a 415 number from the same rate centre both answer "San Francisco, CA" with region_level locality. The United States phone facts link a page for every state with more than one area code, listing each code's overlays and in-service date, and a page of new and planned area codes; Canada's do the same for its provinces.
What it does not mean
An overlay says nothing about who holds a number or when it was issued. A number in the newest code of a complex is not new for that reason alone, and one in the oldest code may have been assigned last week: block_assigned_at dates the number's own code or block where the table dates it. The ten-digit dialling an overlay brings is about how callers dial, not about the number: a lookup reads the ten digits and the eleven with the leading 1, sent with country: "US", and the E.164 form to the same e164.