An Area Code Overlay is an area code that covers the same area as another area code. In North America, each area code has a potential capacity of around 8 Million unique phone numbers (7,919,900 to be exact). Many area codes have exhausted all possible phone numbers, so one or more area codes are added to accommodate.
212 is the original area code for New York City (Manhatan to be exact). The overlays
332 have been added as the need for additional numbers has grown. You can see a full list of overlays in the North American Numbering Plan.
To check if Twilio has available phone numbers in your primary target area code or an overlay, you should check through the console.
In the above image, you can see a search for the area code '209', for the general area around Stockton, CA. If you were searching for a San Francisco number, you could try '
415' or the area code overlay '
628'. Availability will be detailed through results, but Twilio very often will have numbers available from primary area codes and overlays.