Types of Overlays
The North American Numbering Plan Administration recognizes different forms of overlays:
- Distributed overlay (or all-services overlay): an entire existing area gains another area code serving the entire area. Most overlays are of this kind.
- Single concentrated overlay: only the high-growth portion of an existing area gains a second area code.
- Multiple concentrated overlay: the entire existing area gains multiple additional area codes, each of which serves a different subsection of the original. There are no known examples of such being implemented in the NANPA.
- Multiple-area distributed overlay: two or more area codes gain a single new area code covering such an area. Examples include 872 in Chicago, Illinois (over 312 and 773) and 587 in Alberta (over 403 and 780).
- Boundary-extension overlay: a neighboring area code (either an overlay code or single primary area code) is expanded to serve the area as well. Examples include 321 over 407 in central Florida and 778 over 250 in British Columbia.
- Service-specific overlay: the first overlay area code in the NANPA, 917, is the only example of this. It was originally established as an area code specifically for cell phones and pagers in New York City, but soon after, the FCC said area codes going forth could not be service-specific, but they allowed 917 to remain as such. However, 917 is being used for landlines in New York City on a limited basis.
Read more about this topic: Overlay Plan
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