Area Code Overlay
An Area Code Overlay is an area code that covers the same geographical space 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 (Manhattan, to be exact). The overlays 646, 917, and 332 were added later as the need for phone numbers has grown past each code's maximum of 7,919,900 numbers. Here is a full list of overlays in the North American Numbering Plan.
To check if Twilio has available phone numbers in your target area code, or in an overlay, use 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 in the results, but Twilio very often will have numbers available from primary area codes and overlays.