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Telephony Glossary

November 10, 2025•844 words

Table of Contents

ANI (Automatic Number Identification)

Identifies the billing number of the calling party; used for call routing and billing.

Area Code (NPA)

The first three digits of a North American phone number, designating a geographic region (e.g., 207 for Maine).

Auto-Answer

A feature where a device (like a payphone or modem) automatically answers an incoming call after a preset number of rings.

BORSCHT

Acronym describing functions in analog line cards: Battery, Overvoltage, Ringing, Supervision, Coding, Hybrid, and Testing.

Call Detail Record (CDR)

A log of details about a telephone call, including time, duration, origin, and destination.

Carrier

The company providing telephone service; may be an ILEC (Incumbent Local Exchange Carrier) or CLEC (Competitive Local Exchange Carrier).

Central Office (CO)

A local telephone exchange building where subscriber lines connect to the public switched telephone network (PSTN).

CLEC (Competitive Local Exchange Carrier)

A telecom provider that competes with the incumbent carrier using its own or leased facilities.

CPC (Calling Party Control)

A signaling method indicating when the far-end party has hung up; important for payphones and modems.

Dial Tone

The continuous tone indicating a phone line is ready to place a call.

DTMF (Dual Tone Multi-Frequency)

The system used for telephone keypads (touch-tone dialing).

Exchange (NXX)

The three digits following the area code, representing a specific rate center or central office.

ILEC (Incumbent Local Exchange Carrier)

The original local phone company in a given area, often the successor to Bell System or independent telcos.

Intercept Message

The recorded announcement when a number is disconnected or no longer in service (e.g., “The number you have dialed…”).

LATA (Local Access and Transport Area)

A geographic area for dividing local vs. long-distance service in the U.S.

LERG (Local Exchange Routing Guide)

Database that defines which company owns each NPA-NXX and how calls are routed.

Line Card

Hardware module in a switch or PBX that connects an individual telephone line to the system.

Loop Start / Ground Start

Signaling methods between a phone and the switch to start or end calls; loop start is used in most residential lines.

Modem (Modulator–Demodulator)

Device that converts digital data into analog tones for transmission over telephone lines.

NLIS (No Longer In Service)

Industry term indicating a disconnected number or non-functioning line.

NPA-NXX

The first six digits of a North American number (Area Code + Exchange) identifying a rate center and switch.

Off-Hook / On-Hook

Electrical states of a phone: off-hook means in use (loop current present), on-hook means idle.

Payphone (PIP: Public Interest Payphone)

A telephone made available for public use, sometimes subsidized or required by regulators.

POTS (Plain Old Telephone Service)

Traditional analog voice service using copper wires.

Rate Center

A geographic point used by carriers to determine local vs. toll billing and assign phone numbers.

RBOC (Regional Bell Operating Company)

One of the regional carriers created after the AT&T divestiture in 1984.

Ringback Tone

The sound a caller hears while waiting for the recipient’s line to be answered.

SIT (Special Information Tone)

A standardized triple-tone signal preceding intercept recordings (e.g., disconnected numbers).

SS7 (Signaling System No. 7)

Digital signaling protocol used in the public switched telephone network to set up and tear down calls.

Switch (Class 5 Switch)

Telephone exchange equipment that connects subscriber lines locally (e.g., DMS-100, 5ESS).

Tandem Switch

Intermediate exchange that connects multiple local switches for routing long-distance or interoffice traffic.

Tip and Ring

The two wires of a traditional analog phone line: “tip” (positive) and “ring” (negative).

Trunk

A communication line connecting switches or PBXs rather than individual users.

VoIP (Voice over Internet Protocol)

Voice service delivered over IP networks instead of the traditional circuit-switched telephone network.

Wink Start

Supervisory signaling used in E&M trunks and PBX systems, indicating readiness to send digits.

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