NumberGlossaryPhone numbers, clearly explained.
Understand area codes, prefixes, number portability, caller ID, and the terms behind North American phone numbers.
How a number is built
Every North American number has the same parts. Hover or read across — then decode your own.
Common questions
NPA, NXX, thousands-block, line — decode any number.
Number portability, and who actually serves a number.
One changes your number; the other does not.
Why assignment records are not a current-carrier lookup.
Browse by topic
Number structure
How a phone number is built
Area codes & change
Overlays, splits and exhaust
Allocation & carriers
How numbers are assigned
Portability & routing
Keeping a number, and who serves it
Caller ID & services
What shows up, and special codes
Tools
Number decoder →
Break a number into its parts, each explained. The number stays in your browser — it does not identify who owns or is calling from a number.
Compare confusing terms →
Overlay vs split, assignee vs current carrier, and more — side by side, with the tell that separates them.
All 30 terms
Block / Code Status
The administrative state of a prefix or block in the numbering records — assigned, available or retained — read separately for the US (NANPA) and Canada (CNA), whose status sets differ.
Caller ID
The calling-party number (and optionally name) delivered with an incoming call — which can be spoofed.
Caller ID Name CNAM
The name shown with an incoming call. In the traditional U.S. model the receiving carrier looks it up from a database keyed on the calling number; how the name is delivered varies by country and network.
Caller ID Spoofing
Deliberately falsifying the calling number or name transmitted with a call so a different identity shows on the recipient's device.
Central Office Code NXX
The three-digit prefix after the area code (the "NXX" in NPA-NXX-XXXX), historically identifying a telephone switch.
Country Code
The ITU-T E.164 dialing prefix that identifies a country or integrated numbering plan; +1 covers the entire North American Numbering Plan.
Current Service Provider
The carrier that actually serves a telephone number today — which, after porting, may differ from the original assignee.
Direct Inward Dialing DID
A service feature that lets an outside caller reach a specific PBX extension directly, without an operator, by dialing an assigned public number.
E.164
The ITU standard for international telephone number format — e.g. +1 626 555 0100 — with country code and up to 15 digits.
Line Number
The last four digits of a phone number — the subscriber number within a central-office code.
Line Type
The classification of a phone number by how its service is delivered — wireline (landline), wireless (mobile), or VoIP — not a fixed property of the number itself.
Local Access and Transport Area LATA
A regional boundary from the Bell-System breakup that historically separated local from long-distance calling.
Location Routing Number LRN
A ten-digit number that identifies the switch serving a ported number, so calls route to the right network.
N11 Code
A three-digit abbreviated dialing code of the form N-1-1 (211, 311, 411, 511, 611, 711, 811, 911) reserved for special services.
Non-Geographic Code
An area code not tied to a place — toll-free, premium, personal-communications or special-service codes like 500, 710 or 900.
North American Numbering Plan NANP
The telephone numbering plan shared by the United States, Canada and 18 other countries and territories under country code 1 — 25 numbering regions in 20 countries.
NPA Exhaust
The point at which an area code runs out of assignable central-office codes, triggering relief planning.
Number Pooling
Allocating numbers in 1,000-blocks instead of whole NXX codes, so capacity is shared and conserved.
Number Portability LNP
The ability to keep your telephone number when switching carriers — which breaks the link between a number and its original assignee.
Numbering Plan Area NPA
The three-digit area code that begins a North American telephone number (the "NPA" in NPA-NXX-XXXX).
Operating Company Number OCN
A four-character code identifying the service provider a numbering resource is assigned to.
Original Assignee
The service provider a prefix or block was first assigned to — not necessarily the carrier serving a number today.
Overlay
Adding a new area code over the same geography as an existing one, so both serve the region and ten-digit dialing becomes required.
Rate Center
A geographic area used to define local calling boundaries and to which central-office codes are assigned.
Relief Plan
The process of adding numbering capacity to an area code approaching exhaustion — usually an overlay, historically a split.
Split
The older relief method that divides an area code's geography, forcing part of the region onto a new area code.
STIR/SHAKEN
A framework that cryptographically signs caller ID so networks can verify a call really came from the number it claims.
Ten-Digit Dialing
Dialing the full area code plus number for local calls, required wherever an overlay is in place.
Thousands-Block
A pooled block of 1,000 consecutive numbers within an NXX (e.g. 626-470-0XXX) — under pooling, the unit numbers are allocated in.
Toll-Free Number
A number in the 800/833/844/855/866/877/888 codes where the called party, not the caller, pays for the call.
Every definition cites its primary sources and is open to correction — see how this is made. Definitions licensed CC BY 4.0. For live numbering data, see NumberResearch.