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VoIP Glossary


In terms of telephony, Analog refers to any device which is not digital. This is usually in reference to a legacy telephony device.


Analog telephone adaptor is the hardware device that connects a conventional telephone and converts the voice signals into IP packets for use with VoIP. Fax machines are a common use for these devices.

Auto Attendant

Also known as Integrated Voice Response (IVR) and Digital Receptionists, Auto Attendants are used to audibly present a list of options to the caller. Their choice is received by the caller pressing the keys on their phone.

Audio Conferencing

The process of merging two or more callers so they can all communicate together.


Bandwidth is the volume of data that can be transmitted over a communication line in a fixed amount of time. It is expressed in bits per second (bps) or bytes per. ISPs also refer to this as the “Internet Speed” of the connection.

Call Duration

The length of time for a phone call.


The loss of speech-signal components, resulting in the dropping of the initial or end parts of a word or words.

Class 5 (Telephony) Switch

A Class 5 switch, in United States telephony jargon refers to a telephone switch or exchange located at the local telephone company’s central office, directly serving subscribers. Class 5 switch services include basic dial-tone, calling features, and additional digital and data services to subscribers using the local loop. A key part of SIP/VoIP/IMS networks/systems are IP based class 5 switches.


Codec is a term that arises from the Compressor-Decompressor or enCOder/DECoder process. It is used for software or hardware devices that can convert or transform a data stream. For instance, at the transmitting end codecs can encode a data stream or data signal for easy transmission, storage or encryption. At the receiving end, they can decode the signal in the appropriate form for viewing. They are most suitable for videoconferencing and streaming media solutions.


CSID stands for “called subscriber information”. When a fax transmission occurs, the CSID (receiving fax machine’s information) is transmitted to the sending machine. This information can be used in confirmation pages and fax logs.


A Customer Service Record is the record of all services and related information held by a service provider for a client. In cases where there is a dispute or rejection in the LNP process we will often ask for a CSR to confirm the information.

Hunt Group

This is a call routing strategy where the call is routed to a group of users, either simultaneously or in sequence.


This refers to the squeezing of data in a format that takes less space to store or less bandwidth to transmit. It is very useful in handling large graphics, audio and video files.

Conference Bridge

A device used to connect multiple parties for the purposes of group collaboration. Conference Bridges can be audio only or include video. More advanced bridging systems also allow for screen sharing, polls and questions.

Dial Plans

Dial Plans are sequences of characters used to translate dialed numbers into outbound dial strings. Dial Plans can be used as filters; to allow, disallow or manipulate dialed numbers. If a dialed number, in the device, matches a set Dial Plan the device will then transmit the dialed numbers outbound. Dial Plans can be used to prevent calls to certain destinations such as 411 and International Numbers or to add in an area code for 7-digit dialing. Dial Plans look very similar from manufacturer to manufacturer but are not always the same. Most dial plans are based on the (MGCP RFC 3435) Dial Plan but have modifications for various reasons.


The original definition is Direct Inward Dial. However, this is commonly used to refer to telephone numbers that are not toll free.

Dual-Tone Multifrequency (DTMF)

The system used by touch-tone telephones. DTMF assigns a specific frequency (made up of two separate tones) to each key so that it can easily be identified by a microprocessor. This is basically the technology behind touch tone dialing.

Emergency 911 Calls

This is a special telephone number in the United States that handles all calls related to police, fire or medical emergencies. The number, which is allotted under the North American Numbering Plan (NANP), is answered by either a telephone operator or an emergency service dispatcher, who, in turn, alerts the appropriate emergency service.

Find-Me / Follow-Me

A feature that allows calls to find you wherever you are, ringing multiple phones (such as your cell phone, home phone, and work phone) all at once.


Firm Order Completion. In the context of local number porting, this refers to the date that the number will be placed with the new service provider.

Full Duplex

In telephony and data communications, the ability for both ends of a communication to simultaneously send and receive information without degrading the quality or intelligibility of the content.

High-Availability (HA)

Refers to devices or deployment strategies designed to provide access to fully functioning systems at all times. One such strategy is to cluster devices so that the primary device can fail over to the secondary one if necessary.

Internet Congestion

Internet congestion occurs when a large volume of data is being routed on low bandwidth lines or across networks that have high latency and cannot handle large volumes. The result is slowing down of packet movement, packet loss and drop in service quality.


IP, which is the acronym for Internet Protocol, defines the way data packets, also called datagrams, should be moved between the destination and the source. More technically, it can be defined as the network layer protocol in the TCP/IP communications protocol suite.

IP Address

An IP address, also known as Internet Protocol address, is the number used to route to all devices that are connected to the net. Each device has its own unique number which it uses to communicate. The numbers range from to

IP Phone

AKA Internet Phone, SIP Phone or VoIP Phone. An IP phone is one that converts voice into digital packets and vice versa to make phone calls over Internet possible. It has built-in IP signaling protocols such as SIP or H.323 that ensure that the voice is routed to the right destination over the net. On the media side the IP Phone uses audio or/and video codecs such as G.711 or/and H.261 respectively over RTP. The IP phones come with several value-added services like voicemail, e-mail, call number blocking etc.


Internet Service Provider. A business that provides subscriber-based access to the Internet.


International Telecommunication Union, is a telecommunications standards body based in Geneva. It works under the aegis of the United Nations and makes recommendations on standards in telecommunications, information technology, consumer electronics, broadcasting and multimedia communications.


It is a term used to indicate a momentary fluctuation in the transmission signal.


Kilobits per second and is used to indicate the data transfer speed.


Lag is the term used to indicate the extra time taken by a packet of data to travel from the source computer to the destination computer and back again.


Latency is the time that elapses between the initiation of a request for data and the start of the actual data transfer.

Local Number Porting (LNP)

The process of moving phone numbers from one service provider to another.

LSP (Losing Service Provider)

The Losing Service Provider is the company that numbers are being ported away from.

Mean Opinion Score (MOS)

A measurement of the subjective quality of human speech, represented as a rating index. MOS is derived by taking the average of numerical scores given by juries to rate quality and using it as a quantitative indicator of system performance.


In computer telephony, any means of message store and forward. This includes fax mail, voice mail and broadcast messaging.


Multimedia Message service


Stands for North American Numbering Plan. A telephone numbering system that has evolved the way area codes and numbers are allotted. The system was established in 1947 and covers the United States, Canada and a few neighboring areas. It uses a three-digit area code and seven-digit telephone numbers. Its fiat is, however, limited to the public switched telephone networks only.


A logically grouped unit of data. Packets contain a payload (the information to be transmitted), originator, destination and synchronizing information. The idea with packets is to transmit them over a network so each individual packet can be sent along the most optimal route to its. Packets are assembled on one end of the communication and re-assembled on the receiving end based on the header addressing information at the front of each packet. Routers in the network will store and forward packets based on network delays, errors and re-transmittal requests from the receiving end.

Packet Loss

Packet loss is the term used to indicate the loss of data packets during transmission over a computer network. This may happen on account of high network latency or on account of overloading of switches or routers that are unable to process or route all the incoming data.

Packet Switching

A means of economically sending and receiving data over alternate, multiple network channels. The premise for packet switching is the packet, a small bundle of information containing the payload and routing information. Packet switching takes data, breaks it down into packets, transmits the packets and does the reverse on the other end. Packets can be sent in order and then be received in a different order – only to be put back in the correct order in seconds.


Private Branch Exchange. Or PABX (Private Automatic Branch Exchange). In telephony, a PBX system behaves as a customer’s premises over trunk lines (thus the term “branch”). At first, PBXs mimicked a small telephone company switchboard. Users would use an operator to take and make telephone calls to and from the PSTN (Public Switched Telephone Network). Over time, users were able to dial directly, without the use of an operator. Today, computer telephony platforms such as automated attendants are able to route incoming calls automatically, too.

Peer-to-Peer (P2P)

The term peer-to-peer is used to indicate a form of computing where two or more than two users can share files or CPU power. They can even transmit real time data such as telephony traffic on their highly ad hoc networks. Interestingly, the peer-to-peer network does not work on the traditional client-server model but on equal peer nodes that work both as “clients” and “servers” to other nodes on the network.


Point of Presence, equivalent of a local phone company’s central office. The place your long-distance carrier terminates your long-distance lines just before those lines are connected to your local phone company’s lines, or to your own direct hookup.

Alternate Definition: Post Office Protocol. An Internet standard for the storage and retrieval of email messages

POTS (Plain Old Telephone Service)

The typical, familiar model of a single phone line and a single phone number.


It is a convention or standard that defines the procedures to be adopted regarding the transmission of data between two computing end points. These procedures include the way the sending device should sign off a message or how the receiving device should indicate the receipt of a message. Similarly, the protocols also lay down guidelines for error checking, data compression, and other relevant operational details.


Public Switched Telephone Network. The combination of local, long-distance, and international carriers that make up the worldwide telephone network.

QoS (Quality of Service)

The ability of a network (including applications, hosts, and infrastructure devices) to deliver traffic with minimum delay and maximum availability.

Real Time

A communication wherein any perceptible delay between the sender and receiver are minimal and tolerated. Regular telephone calls are real time. Point-to-point fax transmissions are “close” to real time. Voice messaging is in non-real time.


Responsible Organization is the service provider that is charged with receiving calls to a toll-free number.

Sample Rate

This is the number of samples of audio carried per second, measured in Hz or kHz (one kHz being 1 000 Hz). For example, 44 100 samples per second can be expressed as either 44 100 Hz, or 44.1 kHz. Bandwidth is the difference between the highest and lowest frequencies carried in an audio.

Service Provider

An addressable entity providing application and administrative support to the client environment by responding to client requests and maintaining the operational integrity of the server.

SIP (Session Initiation Protocol)

An Internet Engineering Task Force (IETF) standard for initiating, maintaining, and terminating an interactive user session involving video, voice, chat, gaming, virtual reality, and more.

SIP Phone (Also see above IP Phone)

A SIP phone is a telephone that uses the SIP (Session Initiation Protocol) standard to make a voice call over the Internet for signaling (and uses RTP for media). The SIP phones come with several value-added services like voicemail, e-mail, call number blocking etc. There are (normally) no charges for making calls from one SIP phone to another, and negligible charges for routing the call from a SIP phone to a PSTN phone.

SMS (Texting)

Short Message Service


The software installed in the user’s device to make calls over the Internet. This can be on a computer, smart phone or tablet.


It is a software application that is used to keep track of, monitor or regulate connections at the junction point between circuit and packet networks. This software is loaded in computers and is now replacing hardware switches on most telecom networks.


Transmission Control Protocol. The transport layer protocol developed for the ARPAnet which comprises layers 4 and 5 of the OSI model. TCP controls sequential data exchange in TCP/IP for remotely hosts in a peer-to-peer network.


Taken from Greek root words meaning “far sound”, telephony is the discipline of converting or transmitting voice or other signals over a distance, and then re-converting them to an audible sound at the far end.

Toll Free Number (TFN)

In North America these are numbers such as 833, 844, 855, 866, 877 and 888. Unlike long distance calls that are usually billed to the person placing the call, toll free calls are billed to the person receiving the call.

ULC (Underlying Carrier)

All of telecommunications is comprised of various carriers passing traffic amongst eachother. With the current Porting process, even when you “move” a number from one carrier to the other, the original number still remains with the original carrier. The routing table is updated to forward calls to the DID to the new carrier. While you may have VoIP service with FireComm, we utilize various ULCs to pass voice traffic to each from other VoIP providers.

VoIP (Voice over IP)

The process of making and receiving voice transmissions over any IP network. IP networks include the Internet, office LANs, and private data networks between corporate offices. The main advantage of VoIP is that users can connect from anywhere and make phone calls without incurring typical analog telephone charges, such as for long-distance calls.

VoIP Gateway

This device provides the conversion interface between the public switched telephone network (PSTN) and an IP network for voice and fax calls. Its primary functions include: voice and fax compression/decompression, packetization, call routing and control signaling. It also provides an interface to Gatekeepers or Softswitches, billing systems, and network management systems.


VoIP PBX, which stands for Voice over Internet Protocol Private Branch eXchange, is a telephone switch that converts IP phone calls into traditional circuit-switched TDM connections. It also supports traditional analog and digital telephones.

VoIP Phone

A VoIP phone is one that uses the Internet to route voice calls by converting the voice data into IP packets and vice versa. The phones come with built-in IP signaling protocols such as H.323 or SIP that help in the routing of data to the right destination. A VoIP phone can also be a software application that is installed in the user’s PC. In this case it is known as the Softphone. Also, the calls in this case have to be made from the PC, and not through a telephone instrument.

Web Browser

Client software used to view information on Web servers. Can display graphics. Web browsers are also packaged with email clients, newsreaders and in some cases, IP Telephony clients.

Updated on May 28, 2019

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