Glossary
The full glossary lives on this page; each term also has its own anchor page for linking. Definitions are grounded in NANPA and FCC documentation — sources on each term's page.
Caller ID spoofing
Deliberately falsifying the number displayed on caller ID. Spoofing with intent to defraud, cause harm, or wrongly obtain anything of value is illegal under the Truth in Caller ID Act.
ERC — Easily Recognizable Code
An NPA whose last two digits are identical (800, 888, 877, 866, 855, 844, 833, 900...), reserved for special services such as toll-free and premium-rate rather than geographic assignment.
Exhaust (NPA exhaust)
The projected point at which all assignable central office codes in an area code are used up. NANPA publishes exhaust forecasts twice a year; approaching exhaust drives overlay planning.
Jeopardy (NPA jeopardy)
A status NANPA declares when an area code's remaining central office codes may run out before relief (a new overlay or split) can be implemented, triggering rationing procedures.
LATA — Local Access and Transport Area
A geographic region created at the 1984 AT&T divestiture to delineate local versus long-distance carriage. Rate centers are grouped into LATAs; the boundaries still appear in routing data.
LERG — Local Exchange Routing Guide
The industry routing database (now the TRA BIRRDS/LERG product) that maps every NPA-NXX and thousands-block to its serving switch, rate center and carrier. Public NANPA reports expose a subset of this data.
NANP — North American Numbering Plan
The integrated telephone numbering plan covering the United States and its territories, Canada, and much of the Caribbean. Established in 1947, it uses ten-digit numbers in the form NPA-NXX-XXXX under country code +1.
NANPA — North American Numbering Plan Administrator
The neutral administrator that assigns NANP numbering resources — area codes, central office codes, and related resources — under FCC oversight. NANPA publishes the public numbering data this site is built on.
National Do Not Call Registry
The FTC-run registry where consumers list numbers to opt out of most telemarketing calls. Registration is free and does not expire. Scammers ignore the registry, so it mainly reduces calls from law-abiding telemarketers.
NPA — Numbering Plan Area (area code)
The first three digits of a ten-digit NANP number, commonly called the area code. An NPA is either geographic (tied to a territory) or non-geographic (toll-free, premium and other services).
Number portability (LNP)
The right to keep your phone number when switching carriers within the same location. Ported numbers keep their original NPA-NXX in routing data, which is another reason prefix-based location guesses can be wrong.
NXX — Central office code (prefix)
Digits four through six of a NANP number. Each NXX within an area code is assigned to a carrier and anchored to a rate center. N stands for digits 2–9, X for 0–9, so 200–999 are possible prefixes.
OCN — Operating Company Number
A four-character code identifying a telecommunications carrier in numbering and routing databases. Each NXX assignment record lists the holder's OCN.
Overlay
A relief method that adds a new area code to the same territory as an existing one instead of splitting it. Existing numbers keep their code; new numbers may get the new code. Overlays require ten-digit local dialing.
Permissive dialing period
A transition window after an overlay or dialing-plan change during which both the old and new dialing patterns complete calls, before the new pattern becomes mandatory.
Rate center
A geographic point (and surrounding area) that the telephone industry uses to anchor phone numbers for rating and billing local versus toll calls. Every NXX prefix belongs to exactly one rate center; a rate center may be served by several area codes.
Relief (area code relief)
The regulatory process of adding numbering capacity to an exhausting area code, via an overlay or a split, approved by the state utility commission and implemented by the industry with NANPA coordination.
Robocall
A call that delivers a prerecorded or artificial voice message. Telemarketing robocalls to consumers generally require prior express written consent under FCC rules; informational calls (school closings, flight changes) are treated differently.
Split (geographic split)
A relief method that divides an area code's territory into parts, with one part keeping the old code and the others receiving new codes. Splits force number changes and have largely been replaced by overlays.
STIR/SHAKEN
A framework of technical standards (Secure Telephone Identity Revisited / Signature-based Handling of Asserted information using toKENs) that lets carriers cryptographically sign and verify caller ID information to combat spoofed robocalls. US carriers are required by the FCC to implement it in IP networks.
TCPA — Telephone Consumer Protection Act
The 1991 federal law restricting telemarketing calls, autodialers, prerecorded messages, and unsolicited faxes. It is the legal basis for most FCC robocall rules and private robocall lawsuits.
Ten-digit dialing
Dialing the full NPA-NXX-XXXX even for local calls. Mandatory in overlay areas and, since 2021, in many other areas so 988 could be reserved for the Suicide & Crisis Lifeline.
Thousands-block number pooling
Allocating numbers to carriers in blocks of 1,000 (NPA-NXX-X) instead of full 10,000-number prefixes, so scarce numbering resources are used more efficiently. Pooled prefixes are shared among several carriers.
Toll-free number
A number in the 800, 833, 844, 855, 866, 877 or 888 codes where the called party pays for the call. Toll-free numbers are not geographic; they are managed in the SMS/800 database by Somos under FCC rules.
VoIP number
A phone number served over Voice over Internet Protocol rather than the wireline or cellular networks. VoIP numbers use ordinary NANP area codes, so the area code no longer guarantees the caller's location.
Wangiri (one-ring scam)
A callback scam — Japanese for 'one (ring) and cut' — in which the scammer rings once and hangs up, hoping the target calls back an international premium-rate number.