Telco Glossary

A system that automatically delivers a caller's address and callback number to a 9-1-1 dispatcher (PSAP) when a call is placed. ALI databases are maintained by LECs and queried in real-time during emergency calls. Wireless ALI uses GPS or cell-tower triangulation rather than a fixed address.

The telephone number of the originating party, delivered by the network via SS7 signaling. Unlike Caller ID (which can be spoofed), ANI is a billing-grade network identifier used by carriers, IXCs, and 9-1-1 systems. ANI II digits (two additional digits) identify the line type (e.g., hotel, payphone, prison).

Two-digit codes appended to ANI that classify the originating line type. Code 00 = regular wireline; 27 = payphone; 61 = cellular; 63 = restricted; 70 = prison/inmate. These codes help carriers apply appropriate billing and routing rules.

The North American standards development organization for telecommunications. ATIS manages NANPA, NPAC, the LERG database, and oversees industry standards including STIR/SHAKEN, interconnection, and number administration.

NANPA's internal database system used to track telephone number assignments, NPA-NXX assignments, and number resource utilization. Carriers submit NRUF (Number Resource Utilization/Forecast) reports into BIRRDS to justify additional number resources.

A 1994 U.S. federal law requiring telecommunications carriers and equipment manufacturers to build lawful intercept (wiretap) capabilities into their systems. Extended in 2006 to cover broadband and VoIP providers. Carriers must provide a "lawfully authorized electronic surveillance" interface to law enforcement.

A 4-digit code (formerly 3-digit) assigned by ATIS to identify interexchange carriers (IXCs). CICs are used to route calls over the PSTN and are dialed as 10-10-XXXX (10-10 dialing). AT&T's CIC is 0288; MCI/Verizon is 0222; Sprint is 0333.

A telephone company that competes with the incumbent local carrier (ILEC) to provide local telephone service. Created by the Telecommunications Act of 1996, CLECs lease network elements from ILECs (UNE-P) or build their own infrastructure. Examples: Lumen (CenturyLink), tw telecom, Windstream, Cbeyond.

The number presented to the called party as Caller ID. CLID is derived from the ANI or CPN and can be legitimately set by the originating carrier or PBX. Unlike ANI, CLID can be overridden and is subject to spoofing — STIR/SHAKEN exists to authenticate it.

An 11-character alphanumeric code assigned by Telcordia (formerly Bellcore) that uniquely identifies every network element and physical location in North America. The first 4 chars = city/place, chars 5-6 = state/province, chars 7-8 = site ID, chars 9-11 = equipment/switch type. Example: NYCMNYBXDSL = New York City Manhattan (NYCM), NY, Bronx site (BX), DSL equipment.

The name portion of Caller ID — the text displayed alongside the phone number. CNAM is stored in LIDB databases and queried by the terminating carrier. Each carrier's LIDB may have different data, leading to inconsistent name display. Businesses register names through their LEC; CNAM records are distinct from NPAC records.

The physical facility housing telephone switching equipment (Class 4 or Class 5 switches) that connects subscriber lines and routes calls. Each CO serves a defined geographic area and is identified by its NPA-NXX prefix. The local loop — the copper pair from your home — terminates at the CO. Modern COs increasingly use digital remote switching units (RSUs).

The originating telephone number carried in SS7 ISUP signaling messages. CPN is the network-level identifier used to populate Caller ID displays and ANI. It may differ from the Presentation Number or PAI in VoIP environments when call forwarding or spoofing is involved.

A service allowing external callers to reach a specific PBX extension without going through an operator. The carrier assigns a block of numbers (e.g., 200 DIDs) to a business's PBX trunk, and the PBX routes each incoming number to the correct extension. Common in enterprise telephony and VoIP hosted PBX systems.

A suite of IETF standards that adds cryptographic signatures to DNS records to prevent cache poisoning and spoofing. In telecom, DNSSEC is critical for securing ENUM (Electronic Numbering) lookups that map E.164 telephone numbers to VoIP SIP addresses via DNS.

Hierarchical digital transmission standards. DS0 = 64 kbps (one voice channel). DS1 (T1) = 24 DS0s = 1.544 Mbps. DS3 (T3) = 28 DS1s = 672 DS0s = 44.736 Mbps. DS1/T1 lines are the backbone of traditional enterprise and carrier interconnection; DS3/T3 lines connect central offices and IXC POPs.

Broadband internet technology transmitted over the existing copper telephone local loop. ADSL (Asymmetric DSL) provides faster download than upload. VDSL2 can achieve hundreds of Mbps over short distances. DSL coexists with POTS service on the same copper pair using frequency division multiplexing.

The digits of the number actually dialed by the caller, delivered to the terminating equipment (typically a PBX or call center ACD). While ANI identifies the caller, DNIS identifies which number was called — critical when one trunk group carries multiple toll-free or DID numbers.

The ITU-T standard defining the format of telephone numbers worldwide. An E.164 number consists of a country code (CC) + national significant number (NSN), up to 15 digits, typically written with a leading +. For North America: +1 (NPA) NXX-XXXX. E.164 formatting is required in SIP headers and ENUM lookups.

The emergency call system that automatically routes 9-1-1 calls to the appropriate Public Safety Answering Point (PSAP) and delivers the caller's ANI and ALI (address/location). Phase I E911 for wireless delivers cell tower location; Phase II mandates GPS-level accuracy within 50-300 meters. VoIP providers must register a physical address for E911 compliance.

An IETF protocol (RFC 6116) that maps E.164 telephone numbers to internet resources (SIP URIs, email, URLs) using DNS NAPTR records. A lookup for +1-212-555-1234 queries 4.3.2.1.5.5.5.2.1.2.1.e164.arpa in DNS. ENUM enables seamless PSTN-to-VoIP call routing without a centralized lookup registry.

A 32-bit unique identifier programmed into older CDMA mobile phones (pre-3G). ESNs were used by carriers like Sprint and Verizon to authenticate devices on the network. ESNs have been largely superseded by MEIDs (Mobile Equipment Identifiers) for CDMA and IMEIs for GSM/LTE devices.

The U.S. independent government agency that regulates interstate and international communications by radio, television, wire, satellite, and cable. Established by the Communications Act of 1934, the FCC oversees carrier licensing, spectrum allocation, consumer protection, and rules governing number portability, STIR/SHAKEN, and robocall mitigation.

A number routing designator used in NPAC/LNP systems when a ported number's routing information cannot be determined or is in a transitional state. FNRNs prevent routing failures during port-in/port-out windows by providing a fallback routing instruction.

A unique 10-digit identifier assigned by the FCC to every entity doing business with the agency. Required for spectrum licensees, common carriers, cable operators, and entities filing in CORES (Commission Registration System). FRNs are public record and searchable in the FCC's license database.

The established telephone company that historically held a monopoly on local telephone service in a given area. Most ILECs are former Bell Operating Companies (AT&T, Verizon) or independent telephone companies. ILECs own the local loop copper infrastructure and are required by the 1996 Telecom Act to lease network elements to CLECs.

A centralized database concept (analogous to a HLR in mobile networks) used in the North American LNP architecture to track where a ported telephone number currently resides. The NPAC serves as the authoritative ILR for the NANP region, providing real-time porting data to all carriers.

A unique 15-digit identifier stored on a SIM card that identifies a mobile subscriber to the network. Format: MCC (3 digits, country) + MNC (2-3 digits, network) + MSIN (subscriber). The IMSI never appears to end users — it's the network-level identity, while MSISDN is the dialable phone number assigned to that SIM.

The physical and contractual arrangement by which two telecommunications networks exchange traffic. Interconnect agreements govern how calls hand off between carriers, including technical interface standards (SS7, SIP), compensation terms (access charges, bill-and-keep), and routing obligations.

A digital circuit-switched telephone network standard from the 1980s-90s. BRI (Basic Rate Interface) = 2B+D (2 x 64 kbps voice + 1 x 16 kbps signaling). PRI (Primary Rate Interface) = 23B+D in North America = one T1 line. ISDN PRI remains in use for legacy PBX connections despite being largely supplanted by SIP trunking.

A long-distance telephone carrier that carries calls between LATAs (inter-LATA) and across the PSTN backbone. Historic IXCs include AT&T, MCI/WorldCom, and Sprint. Customers select their IXC via PIC (Primary Interexchange Carrier) selection. IXCs are distinguished from LECs, which handle local service within a LATA.

A geographic region within which a local exchange carrier (LEC) may carry telephone calls without involving a long-distance carrier. Created as part of the 1984 AT&T divestiture, there are 162 LATAs in the NANP region. Calls within a LATA (intra-LATA) are handled by the LEC; calls crossing LATA boundaries (inter-LATA) require an IXC.

The generic term for a telephone company providing local telephone service, encompassing both ILECs (incumbents) and CLECs (competitors). The LEC owns or leases the last-mile connection to customers and operates central offices within a LATA. LECs collect access charges from IXCs for originating and terminating long-distance calls.

The industry's master routing database, published monthly by Telcordia (Iconectiv). The LERG maps every NPA-NXX code to its serving switch (CLLI), LATA, OCN, rate center, and routing information. Used by all carriers for call routing decisions. Access requires an industry subscription; NANPA publishes a subset of NPA-NXX data publicly.

A network database queried in real-time to retrieve CNAM (calling name) information and to validate billing for collect calls and third-number billing. LIDBs are maintained by carriers and interconnected via SS7. When you see a name on your Caller ID, it came from a LIDB query made by your terminating carrier. There is no single national LIDB — each major carrier maintains its own.

The FCC-mandated capability allowing subscribers to keep their telephone number when switching carriers, moving within a rate center, or changing service type (wireline to wireless). Implemented in 1996-97, LNP is managed through NPAC (the Porting Administration Center). When a number is ported, its routing information in NPAC and carrier databases is updated, redirecting calls to the new carrier.

The full international telephone number of a mobile subscriber — what most people call "their cell phone number." Format follows E.164: CC + NDC + SN. In North America, the MSISDN is simply +1 (NPA) NXX-XXXX. The MSISDN is the dialable number; the IMSI is the internal network identity stored on the SIM.

The neutral administrator responsible for managing the North American Numbering Plan (NANP), which covers the U.S., Canada, and 18 Caribbean nations under country code +1. NANPA assigns NPA (area codes) and NPA-NXX codes, tracks number utilization via NRUF/BIRRDS, and coordinates new area code overlays and splits. Currently administered by Somos (formerly Neustar), contracted by the FCC.

A generic term for databases containing telephone number metadata such as carrier assignment, line type, porting status, and geographic data. Commercial NIDB providers (e.g., Neustar, Iconectiv, Twilio Lookup) aggregate NANPA, NPAC, LERG, and LIDB data to provide real-time TN intelligence for fraud prevention, routing, and analytics.

The central clearinghouse for number portability data in North America, operated by Somos. When a subscriber ports their number, the gaining carrier submits a port request to NPAC, which coordinates with both carriers and updates the authoritative routing record. All carriers query NPAC (via SV — Service Values) to determine where a ported number should route.

The local copy of NPAC data maintained by each carrier in their network. When a call arrives for a potentially ported number, the carrier's switch performs a real-time dip into the NPDB to obtain the current routing number (NRN or LRN) and route the call to the correct destination carrier. NPDBs are updated in near-real-time from NPAC broadcasts.

The 3-digit area code in a NANP telephone number (NPA-NXX-XXXX). The first digit of an NPA is always 2-9; the original NPAs (1947) used a 0 or 1 as the middle digit to distinguish them from NXX exchange codes. NPAs are assigned by NANPA and may be split or overlaid when number resources are exhausted. There are currently over 370 active NPAs in the NANP.

The 6-digit combination of area code (NPA) and central office exchange code (NXX) that identifies a specific block of 10,000 telephone numbers (0000-9999). NPA-NXX assignments are the fundamental unit of number resource management — each is assigned to exactly one carrier and rate center by NANPA. Approximately 70,000+ active NPA-NXX codes exist in the NANP.

Semi-annual reports filed by carriers with NANPA documenting how many numbers they've assigned to subscribers vs. available in their inventory, along with demand forecasts. NRUF data drives NANPA's decisions about when to assign new NXX codes and when to initiate area code relief planning. High utilization triggers new code assignments.

The 3-digit exchange code (digits 4-6) in a 10-digit NANP number. "N" represents any digit 2-9 (never 0 or 1, which are used for special services); "X" is any digit 0-9. NXX codes within an NPA are assigned by NANPA to carriers in conjunction with rate centers. The NPA-NXX combination was historically tied to a specific central office building before number portability.

A 4-character alphanumeric code assigned by Telcordia (Iconectiv) to identify every telephone company in North America. OCNs appear in the LERG next to each NPA-NXX assignment and identify the carrier responsible for that number block. OCNs map to company names and LEC types (CLEC, ILEC, wireless, etc.).

Origination is the carrier service that picks up a call from a customer or device and injects it onto the PSTN or VoIP network. Termination is the service that delivers a call to its final destination. Wholesale origination and termination are distinct services priced separately — a call typically traverses multiple origination and termination carriers before completion.

A legacy routing code used in the PSTN routing tables (LERG/switch routing) to designate operator services traffic. "OZZ" entries in routing guides indicate calls that should be delivered to an operator services platform rather than a standard terminating switch. Primarily a concern in legacy SS7 routing configurations.

A SIP header (RFC 3325) used between trusted network entities (carriers) to convey the authenticated identity of the originating user. Unlike the SIP From header (which end users can set), PAI is inserted by the originating carrier and carries billing-grade authority. PAI is a key component in STIR/SHAKEN attestation.

A private telephone switching system used within a business or organization to route internal calls and share external telephone lines. Traditional PBXs connect to the PSTN via analog lines, T1/PRI, or SIP trunks. Modern IP-PBXs (like Asterisk, FreePBX, Cisco CallManager) use VoIP internally. A hosted/cloud PBX is called a UCaaS platform.

A bilateral agreement between carriers to exchange traffic directly without involving a third-party transit carrier. Peering arrangements are typically settlement-free (bill-and-keep) between carriers of similar size. VoIP peering uses SIP trunks; PSTN peering uses SS7 interconnects. Carriers that don't peer must use transit to reach each other's customers.

PORTIN is a number portability transaction where a carrier (the gaining carrier) requests that a subscriber's number be moved from another carrier (the losing carrier). PORTOUT is the same transaction from the losing carrier's perspective. Regulated timelines: wireline ports must complete in 1 business day; wireless in 1 hour after validation.

Traditional analog voice telephone service delivered over copper wire pairs — the technology that defined telecommunications for over a century. POTS lines carry voice as analog electrical signals between 300-3400 Hz bandwidth, switched through central office equipment. The FCC has been managing the POTS-to-IP transition since the 2010s as carriers retire copper networks.

Telephone numbers (900/976 in North America) where the calling party is billed above the standard rate for accessing information or entertainment services. The 900-number industry peaked in the 1990s. Premium rate services are regulated by the FCC and must include mandatory disclosure of rates before charges begin.

A 9-1-1 call center staffed by dispatchers who answer emergency calls and coordinate police, fire, and EMS response. PSAPs receive ANI (caller's number) and ALI (location) from the telephone network via E911 systems. There are approximately 6,000 PSAPs in the U.S., ranging from county-wide centers to municipal operations.

The global aggregate of circuit-switched telephone networks operated by national carriers. The PSTN is the infrastructure of physical copper wires, fiber optic cables, microwave links, satellites, and switching equipment that collectively enables telephone calls worldwide. VoIP calls that originate or terminate on a regular phone number pass through PSTN gateways.

One of the seven telephone companies created by the 1984 AT&T divestiture: Ameritech, Bell Atlantic, BellSouth, NYNEX, Pacific Telesis, Southwestern Bell (SBC), and US West. Colloquially called "Baby Bells," RBOCs held local telephone monopolies within their regions. Through mergers, all seven eventually became part of AT&T, Verizon, or CenturyLink.

Codes in carrier billing and routing systems that classify calls for rating and settlement purposes. RMC/RMD data appears in call detail records (CDRs) and is used for inter-carrier compensation, access charge billing, and traffic studies. The codes reflect LATA boundaries, call types, and carrier agreements.

The dominant signaling protocol for initiating, modifying, and terminating real-time communication sessions over IP networks (IETF RFC 3261). SIP handles call setup, teardown, and negotiation of media parameters (via SDP). SIP trunks connect IP PBXs to the PSTN, replacing ISDN PRI. SIP is also the foundation for WebRTC and modern UCaaS platforms.

A unique identifier assigned to each carrier participating in the NPAC/LNP system. SPIDs are used in all porting requests to identify the gaining and losing carriers. Not to be confused with ISDN SPIDs (Service Profile Identifiers), which serve a different purpose in BRI ISDN configuration.

The global standard for telephone network signaling, used to set up and tear down most phone calls and to exchange information across carriers. SS7 protocols include ISUP (call control), TCAP (database queries), and MAP (mobile network management). SS7 vulnerabilities discovered ~2008 allow attackers to intercept calls and texts — a significant ongoing security concern for the telecom industry.

A framework of technical standards (RFC 8224, RFC 8225) for authenticating Caller ID on VoIP calls to combat illegal spoofing and robocalls. The originating carrier signs a call with a cryptographic "passport" (a JSON Web Token) with one of three attestation levels: A (full), B (partial), or C (gateway). The terminating carrier verifies the signature and displays a "verified call" indicator. Mandated by the FCC's TRACED Act (2019), with compliance required for large carriers by June 2021.

T1 (DS1): a digital transmission line carrying 24 voice channels (DS0s) at 1.544 Mbps, the workhorse of enterprise and carrier-grade voice connectivity. T3 (DS3): 28 T1s = 672 voice channels at 44.736 Mbps, used for inter-office trunks and internet backbone. Both use time-division multiplexing (TDM) over copper or fiber.

An intermediate switching facility that routes calls between central offices without serving end subscribers directly. Tandems (Class 4 switches) sit above the local CO (Class 5) in the PSTN hierarchy and handle inter-LATA, intra-LATA, and access tandem traffic. Access tandems connect local exchanges to IXC points of presence. Most tandem functionality is now handled by IP/SIP-based softswitches.

The traditional 5-level PSTN hierarchy classifies switches by function. Class 5 (End Office/Central Office) directly serves subscribers — your local phone line terminates here. Class 4 (Toll Center/Tandem) switches aggregate traffic from multiple Class 5 offices for inter-office routing. Classes 1-3 handled long-distance hierarchy. Alcatel-Lucent 5ESS and Nortel DMS-100 are the dominant Class 5 platforms; Nortel DMS-250 is a Class 4.

In industry parlance, "TN" specifically refers to the full 10-digit subscriber number in the NANP format (NPA-NXX-XXXX). Used in carrier systems, porting requests, LIDB queries, and number management databases to distinguish the specific subscriber-assigned number from the NPA-NXX block assignment.

Numbers in the 800, 888, 877, 866, 855, 844, 833, and 822 NPAs where the called party (subscriber) pays the call charges rather than the caller. Toll-free numbers are managed via the SMS/800 database by Somos and assigned through RespOrgs (Responsible Organizations). They are portable across carriers and route to any termination point nationwide.

A wholesale carrier service where one carrier (the transit provider) routes calls between two other carriers that don't have a direct interconnect or peering agreement. Transit providers aggregate traffic from many smaller carriers and provide reach to networks that would otherwise be unreachable. Unlike peering, transit is typically a paid service.

Technology for transmitting voice communications over IP networks (internet or private IP). VoIP converts analog voice to digital packets transmitted via RTP (Real-time Transport Protocol), with SIP handling call signaling. Consumer VoIP (Vonage, Google Voice), enterprise SIP trunking, and carrier backbone are all VoIP. Calls to/from the PSTN pass through VoIP-PSTN gateways (Class 4/5 softswitches).

Wireline refers to traditional fixed-line telephone service delivered over physical cables (copper, fiber). Wireless refers to mobile cellular service transmitted over radio spectrum. The distinction matters for LNP (porting timelines differ), CNAM lookup reliability, E911 location accuracy, and FCC regulatory treatment. VoIP occupies a hybrid category, often classified by the FCC as "interconnected VoIP."