NPA/NXX is the North American Numbering Plan's name for the first six digits of a US or Canadian number: the three-digit area code (Numbering Plan Area) and the three-digit central office code (NXX). Numbers are allocated to carriers in blocks keyed on those six digits, and the regulators publish the tables. Most US codes are pooled: split again into blocks of a thousand (NPA-NXX-X) that other carriers can hold, which NANPA publishes too.
The central office code
The NXX is the central office code, which NANPA defines as "the second three digits (also called digits D-E-F) of a ten-digit telephone number in the form of NPA-NXX-XXXX" (https://www.nanpa.com/numbering/co-codesthousands-blocks, accessed 27 September 2026). NANPA notes that it is also known as the exchange or the prefix. A code is ten thousand numbers, assigned to one carrier in one rate centre or pooled into thousands-blocks that carriers there hold separately. Not every code can be assigned. N11 digits such as 211, 311 and 911 are dialled on their own as short numbers, and in NANPA's code file of 26 September 2026 no assigned code starts with 0 or 1 or ends in 11; the file marks the N11 codes and 555 unavailable, so a number in one answers block_status: reserved. Not every NPA is a place either: the toll-free codes 800, 833, 844, 855, 866, 877 and 888, the premium-rate 900 and several 5XX personal-number codes carry a service rather than an area, answer is_geographic: false and are typed by line type.
What Spaw answers
The block behind a number sets three fields. block_status says what the table records — allocated, unassigned or reserved; a reserved block, such as the 555 code, raises the reserved_block signal. carrier names the block holder as the regulator lists it; in a pooled US code, the holder of the number's own block of a thousand where NANPA lists one. line_type_source says block_allocation when the block, rather than the numbering plan, settled the line type.
That last one is the whole reason the tables are carried. The plan itself cannot separate American mobiles from landlines, because they share blocks; it can only answer fixed_line_or_mobile. The block holder often settles it — a wireless carrier's block is mobile, a virtual-number wholesaler's is VoIP and raises the virtual signal.
What it does not know
A block describes the original allocation, never a later port. A number moved to another carrier keeps the block it was born in, so carrier is who the numbers were given to and not who serves the handset today.
Every area code with published blocks has a page of its own under the phone lookup, and the plan as a whole is summarised on the phone facts pages for the United States and Canada.