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A-Z of telecoms

A-Z of telecoms


Liam Spry
January 23, 2024

It’s no secret that the ICT and telecoms space is crammed with acronyms.

This is all down to the complex and evolving nature of the technology and the standards in our sector. But that doesn’t mean telecoms should be tricky to understand.

In this blog, I share a comprehensive A-Z telecoms glossary. I’ve broken down over 100 acronyms and technical terms that pop up in our industry day to day, into plain speaking English.

Whether you’re a seasoned pro looking to brush up, or a complete newbie to telecoms, I hope you’ll find my glossary helpful.

If you’re searching for a specific term and can’t find it here, get in touch and I’ll happily update the glossary for you.

My team and I help and support a lot of IT resellers who are new to telecoms. If you feel like you’d benefit from training, we have programmes designed to help. Get in touch to find out more.

Liam Spry's A-Z of telecoms

Liam Spry, Service Desk Team Leader at eve Networks

After building extensive experience as our Support Engineer, Liam stepped into his current management role in August 2023.

Liam now heads up the friendly Service Desk team, who are here to help and support our Partners.


ACD – Automatic Call Distribution 

Allows all incoming calls to be distributed equally amongst a group of people. Typically used in a call centre where operators (agents) log in to make or receive calls. ACD systems provide facilities for monitoring the agents’ performance and the performance of the call centre as a whole. An incoming call will be automatically routed to the first available agent, whether that is an agent who has been free the longest, or an agent that has just become free. 

ADSL – Asymmetric Digital Subscriber Line – Broadband

Transforms the existing twisted copper pairs between the telephone exchange and the telephone socket into a digital line that is solely for access by the subscriber. The speed of transmission is not the same in both directions; downloading is faster than uploading. ADSL will be retired with the PSTN switch off in 2025, see our live blog for further information.

Alpha tagging

The assignment of an alpha-numeric name to a facility. For example, when called by an extension, your phone can display the name of the caller rather than the extension number. When an incoming DDI call is received, a name can be shown that relates to the number that was dialled, enabling one person to answer calls in a variety of different ways e.g. in the names of different companies. 

Analogue line

A single telephone line for used for analogue devices and broadband. This is legacy technology operated as the PSTN network and will be switched off in December 2025. See our live blog for further information.

Analogue device

A device that can be attached to an ordinary analogue telephone line, such as a telephone, fax machine, cordless phone, answering machine, modem etc. When switching to an IP or digital Internet connection some analogue devices can still be used via an ATA (analogue telephone adapter). 


A voicemail feature that allows callers to be automatically transferred to extensions or departments by dialling digits. A tone-dialling phone is normally required to do this. For example, ‘press 1 for Sales, 2 for service’ etc. Within eve Voice, this is known as ‘calling menus.’


The speed at which a circuit can carry data. The more bandwidth, the faster the data transfer, and the lower the costs. Usually measured in Mbps (Megabits per second) or Gbps (Gigabits per second). 

Bits and Bytes

A bit is the smallest unit of information that a computer can store. It can be either 0 or 1, which means on or off. A byte is a group of eight bits that can store more information, such as a letter, a number, or a symbol. For example, the letter A is stored as the byte 01000001, which is made of eight bits.


A type of wireless transmission with a relatively short range (10 – 100 metres). This can be used to make a wireless connection between your telephone, computer, car, headset etc. 


A bolt-on is an additional service that can be added to your main license or tariff. For example, with eve Voice services there are bolt-on services for longer call recording storage or advanced call queue features.  

BRI – Basic Rate Interface

Abbreviated to BRI or ISDN2. An ISDN circuit provides 2 x 64 kbit/sec bearer channels for use by data or speech and one 16 kbit/sec control channel. Two independent calls can be carried at the same time on one BRI circuit. You will also associate this with telephone systems (PBX) as they will require the correct interface card according to the line type (ISDN2 BRI or ISDN30 PRI). 

BLF – Busy Lamp Field

Visual indication of the status of lines or extensions through LEDs. Lights indicate if someone’s extension is busy.


This is effectively a packaged deal. For example, with eve Voice you can get inclusive call minutes packages bundled with most license types. See our licence table for more information.

‘Bundle’ is also an Ofcom definition, defined as a contract, or two or more closely related or linked contracts, between the provider of a Public Electronic Communications Service and an End-User, which: (a) relates, or together relate, to the provision of at least one of the following: (i) an Internet Access Service; or (ii) a Number-based Interpersonal Communications Service; and (b) also relates, or together also relate, to the provision of at least one of the following: (i) another service falling within paragraph (a)(i) or (ii); (ii) any other Public Electronic Communications Service; (iii) an Information Society Service; (iv) a Content Service; and/or (v) Terminal Equipment.

Byte, Kilobyte (KB), Megabyte (MB) & Gigabyte (GB) 

A measure of data. In Telecoms the definitions used are based on one Kilobyte (1KB) equal to 1024 bytes. One Megabyte (1MB) is the same as 1024 Kilobytes (KB) and one Gigabyte (1GB) is 1024 Megabytes (MB). Beyond that there are terabytes (TB), petabytes (PB) and more.

Note: In other technologies (e.g. data storage) Kilobytes can also mean 1000 bytes (and Megabytes equal to 1000 KB, Gigabytes equal to 1000 Megabytes etc.).

Call barring

The prevention of calls to certain destinations e.g. overseas calls or calls to premium rate numbers may be barred. This can be applied to the telephone system or at the network level on the line. This feature is available with eve Voice.

Call forwarding

Also known as call diversion. By dialling a code, an extension user can divert incoming calls to another destination. The destination may be another extension, a group of extensions, the operator, or an external number, for example, a mobile phone. Different types of diversion are usually possible, e.g. Diversion of All calls, Diversion on Busy, Diversion on no reply. 

Call logging

Data recorded about calls made or received through a telephone system. This data can then be used for reporting. This data is available within the eve Voice portal.

Call management

The use of specialist software to analyse and report on call records, which are output from a telephone system and recorded on a computer disk. The results can identify misuse, allocate costs to departments and verify the adequacy of resources. 

Call park

A call can be parked by one user and then retrieved by another. Particularly useful when loudspeaker announcements are made, e.g. “Telephone call. Joe Bloggs dial 811”. If Joe goes to any phone and dials 811 he will get the call that has been parked there for him. 

CDR – Call Detail Record 

A digital file containing call records including CLI, date, time, number dialled, destination, duration and cost. A CDR will be used to create and analyse bills. 


Mobile, Voice or Connectivity network service provider e.g. eve Networks,  BT, TalkTalk etc. Carriers own their infrastructure or network. 

Cat 5/Cat 6

Abbreviation for Category 5. Strictly speaking, this is a specification for the transmission performance of a data cable. However, it is commonly used to describe a building’s cabling system that allows the user to easily route voice and data circuits to any wall socket. It is designed to be network-independent and to allow different computer and telephone systems to co-exist on the same cabling. 

CCU – Central Control Unit

The box or cabinet housing the central equipment that controls the telephone system. 

Centrex – Centralised Exchange

Centrex is a hosted telephony service that operates over a managed broadband connection delivering the functionality of a PBX without many of the costs and manageability issues. The UK’s main solution is BT’s Featureline.


Coder / decoder – A technique used to compress/decompress speech or audio signals. 

CLI – Calling Line Identity

The capture of the caller’s number. The CLI is associated to the line and is commonly known as the ‘telephone number’. The CLI can be presented on outbound calls and withheld. The user can also use an alternative CLI to present on their calls. 

CLIP – Calling Line Identity Presentation

A service that provides a called party with the calling line ID of the caller.

CLIR – Calling Line Identification Restriction

Would stop your own CLI (telephone number) from being presented to the called party. 

Cloud Phone System 

A cloud phone system is where all the functionality of the phone system runs on cloud computers rather than on an on-premise telephone system (see PBX) and users or telephone extensions connect to it via a private network or the Internet. They offer far more flexibility and resilience, see eve Voice for an example of a Cloud Phone System.

COLP – Connected Line Presentation 

A service that provides the caller with the identity of the person he has connected to. For example, you may dial 01234 567890 but that number may be diverted to another. COLP will provide you with the identity of the person you have actually connected to. The identity is typically the telephone number of the connected party. This is a paid-for service from the network provider and compatible equipment is required to make use of it. 


The joining together of more than two telephone users in a single call.

Contact centre

A progression of the call centre merging customer calls with other media such as internet and email in conjunction with CRM applications. A unified approach to customer contact, improving customer service levels leading to increased customer acquisition, satisfaction and retention. 

CPE – Customer Premise Equipment

Equipment that resides on the customer premises for the service to operate, e.g. a router for an Ethernet Fibre service.  

CRM – Customer Relationship Management

A software application to deliver a single view of the customer. Customer records and information relating to the customer’s account are recorded in the CRM. Examples are Salesforce and Oracle. 

DDI – Direct Dial Inward

An inbound or outbound number assigned to a specific handset. The DDI will be associated with the main number (CLI) and is available on ISDN and VoIP.

DECT – Digital Enhanced Cordless Telephony 

A technology that provides greater clarity and smaller handsets for cordless phones. 

Delayed ringing

A feature often used to provide an overflow if the switchboard operator is busy or absent. Incoming calls are sent to the operator but other extensions have delayed ringing, so they will start ringing if the call is not answered after a pre-set time. 

DIA – Direct Internet Access 

A dedicated Ethernet fibre connection that connects to the Internet rather than to a private network. 

DQ / DQR – Directory Enquires

118 services. 

DSS – Direct Station Selector

A unit that fits alongside a telephone to turn it into an operator console. Typically contains a large number of programmable buttons that can be used to call and to indicate the status of extensions (stations). Within eve Voice this feature comes as Expansion Modules.

ECP – Exceptional Call Protection 

ECP is eve’s proprietary fraud protection service. 

EFM – Ethernet in the First Mile

An access technology delivering dedicated synchronous and uncontended bandwidth to a site over multiple copper MPF pairs.  BT Wholesale’s EFM has reached end of life, services were switched off in November 2023.

Ethernet fibre

An access technology delivering dedicated synchronous and uncontended bandwidth to a site over a 100Mpbs, 1Gbps or 10Gbps fibre bearer circuit. This bandwidth can connect a site to the Internet, used for dedicated site-to-site (point to point) or to a private network. 

Fixed Line

Fixed line is often the term used when referring to a solution or product that uses the physical fibre or copper network as opposed to over-the-air technology such as 5G or Wi-Fi.  

FTTC – Fibre to the Cabinet

A part-fibre broadband access technology where fibre runs to the local cabinet,  typically green street cabinet, that then uses legacy copper to link the cabinet to the user premises.  

FTTP/FTTH – Fibre to the Premises/Fibre to the Home

High-speed broadband access technology where fibre connectivity runs all the way to the building, in commercial settings referred to as FTTP and for residential users as FTTH.  

GPS – Global Positioning System

This technology uses a GPS receiver in the phone to calculate from the orbiting satellites your exact location on Earth. Can be used with mapping software on a smartphone to provide a navigation service. 

Group ringing

A group of extensions is rung by dialling a number. The group may be set as a ring group, in which case all of the extensions ring at once, or it may be set as a hunt group, in which case the system will find a free extension in the group to take the call. 

GSM – Global System for Mobile Communications

The mobile phone platform used in Europe and much of the rest of the world. 

Hosted voice 

A term used for cloud services where equipment is hosted on another premise such as a data centre. For example, hosted PBX will be accessed through user licenses paid by subscription. The handsets will be on the customer premises but the features and functionality will come from the platform hosted in a data centre. eve Voice is the hosted voice solution available from eve Networks.

Hot spot

‘Hot spots’ are locations where mobile phone users can connect to the internet using WiFi via a wireless access point.  

Hunt groups

A means of finding a free extension to take a call. Calls are directed to a hunt group which will search for a free extension to take the call. Various hunting types are available: first free or terminal hunting will search for the first free extension in the group, so this person gets most of the calls. Circular or UCD (Uniform Call Distribution) hunting will share calls equally over the group. 


A unique 15-digit serial number that identifies a mobile phone. You can normally display this by typing *#06# on your phone. 

Instant messaging 

A service that allows you to ´chat´ with another internet user in real time, by sending messages to each other. Examples include iMessage, WhatsApp etc.

IP address

A series of characters that uniquely identifies the terminal equipment, which is the origin or destination of data being transmitted. IPv4 is the most widely used but now an old variant with 32 bit address (an example is and the emerging standard is IPv6, which is a 128 bit address (for example 2001:0db8:85a3:0042:1000:8a2e:0370:7334). IPv6 allows for a worldwide pool of 3.4×1038 addresses, or more than 7.9×1028 times as many as IPv4 addresses. IPv4 is a pool of 4,294,967,296 addresses which is almost all used. 

ISP – Internet Service Provider

An organisation set up in business to connect a consumer or business to the internet. 

ISDN – Integrated Services Digital Network

Digital lines over the PSTN. ISDN lines come in 2 flavours: ISDN2 – offering 2 channels of voice. If more channels are required, several ISDN2 bearers can be presented over one number; ISDN30 – ultimately a 2mb circuit divided into 30 channels. A minimum of 8 channels must be open; you can then increase the capacity 2 channels at a time without installing any more bearers until you reach 30. ISDN is being phased out with the PSTN network, see more on our live blog.

ITSP – Internet Telephony Service Provider

A service provider of Voice over IP (VoIP).

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LAN – Local Area Network

Enables PCs to communicate data between each other and common devices or servers also connected to the network usually within the same building.

LOA – Letter of Authority

Industry-regulated authority letter signed by the customer to allow their service provider to port their number to an alternative Network (see ‘Porting’). Sometimes referred to as a Customer Letter of Authority (CLOA). 

Local area code number

Using the same network and technology as the NGN or NTS but with a geographic presence. The numbers will contain the typical STD codes but have the flexibility of not being on the PSTN. For example, a taxi firm may be in Glasgow and want to have a presence in Edinburgh, therefore they will have an Edinburgh number terminated to their office in Glasgow. These numbers are inbound only so do not have any equipment or lines associated with them. It is a cloud service. 


A reference to the location where voicemail or email messages for a particular user are stored. 

MAPI – Messaging Application Protocol Interface

An API for Microsoft Windows which allows programs to become email-aware

MPLS – Multi Protocol Label Switching

Multiprotocol Label Switching (MPLS) is used to connect a company’s disparate sites over a private IP. Using a mechanism in high-performance telecommunications networks, MPLS directs data from one network node to the next based on short path labels rather than long network addresses, avoiding complex lookups in a routing table. 

Music on hold

An audio signal that is played to a caller on hold to reassure him that he has not been cut off. Typically music, it may be interspersed with advertising messages or could be simply a reassuring beep played every few seconds. 

Network switch

A device allowing multiple devices to access a network. The switches come in 8 port, 16 port, 32 port and 48 port. 

NGN – Non Geographic Number

Numbers that are not geographically located, typically 0300, 0500, 0800, 0808, 0844, 0845, 0870, and 0871. Premium rate numbers are also classed as NGN but priced more aggressively. These numbers are inbound only so do not have any equipment or lines associated with them. It is a cloud service. 

Night service

Most telephone systems have at least two operating modes: day service and night service. These are typically used to route incoming calls to a different destination and to apply call barring to prevent unauthorised use of the phones out of hours. 


The Office of Communications in the UK communications regulator overseeing telecommunications, broadcasting and postal industries. Find out more at Ofcom.org.uk.

PBX / PABX – Private Branch Exchange / Private Automated Branch Exchange

On-premise telephone switching system that interconnects telephone extensions to each other, as well as to the outside telephone network. 

PAC or PAC Code – Porting Authorisation Code

A PAC code allows you to transfer an existing mobile phone number from one mobile phone network provider to another mobile phone provider so you can keep your existing mobile phone number. The process is termed mobile number portability, or number porting. 

Porting / number porting

Porting is the name given to the process of transferring a phone number from one network provider to another network provider. Porting allows you to keep your number and is used for both fixed-line and mobile numbers. The process requires a LOA (letter of authority) from the end user. 

PSTN – Public Switched Telephone Network

The world’s collection of interconnected voice-oriented public telephone networks, both commercial and government-owned (AKA a traditional telephone network). In the UK the PSTN network will be switched off at the end of December 2025, get updates via our live blog.

PoE – Power over Ethernet Switch

A device that has several ports available. The switches come in 8 port, 16 port 32 port and 48 port. These switches also supply electricity to the device. See ‘network switch’ – it is the same except a POE also has an electricity supply. 

QoS – Quality of Service

A service level defined by a service agreement between a network user and a network provider, which guarantees a certain level of bandwidth and data flow rates. 

RID – Reseller Identification  

Reseller Identification code allocated by Ofcom, three-character alphabetic codes which are used in the Notification of Transfer (NoT) switching process and can be used to identify a reseller of telecoms services.  


Routes data traffic. Routers can be wireless or wired. The router is the device that sits between the Internet connection and your device. 

RTP – Realtime Transport Protocol

The method of transporting voice over a data connection. 

Screen popping

The initialising and presentation of database information selected using the CLI. Reduces the time spent searching for customer service history etc. When a call comes into the telephone system, the agent’s PC screen will ‘pop’ up the customer records. Typically seen in inbound call centres. 

SIP – Session Initiation Protocol

A signalling protocol, widely used for voice and video calls over the internet. SIP trunks will typically be connected to a telephone system. 

SLA – Service Level Agreement 

An SLA is an agreement concerning a measurable level of service between the service provider and the service receiver. 

SoGEA – Single Order Generic Ethernet Access 

An IP-only service that runs over the same technology as FTTC but does not require a separate analogue or landline. 

SoTAP – Single Order Transitional Access Product 

An IP connection service designed to bridge the gap between legacy WLR-based services and fibre services. It is only available where fibre services are not available, typically where it is not economically viable to replace the existing copper infrastructure with fibre.  

TAPI – Telephony Application Program Interface

A standard devised by Microsoft Corporation for communication between a computer terminal (workstation) and a telephone extension. There are several versions of the TAPI standard and various options within the standard so it should not be assumed that all the features of one TAPI-compliant device will be available on any other TAPI-compliant device. 

TCP / IP – Transmission Control Protocol / Internet Protocol 

Set of communication protocols used to interconnect network devices on the internet or other networks. 


Tethering describes the process of connecting a phone to a computer so that the computer can access the internet via the mobile network.  

Tone dialling

Also known as MF (multi-frequency) or DTMF (dual-tone multi-frequency). A dialling method which generates audio tones when digits are dialled. These tones are sent down the line and can be detected by telephone systems or other devices. 


A telephone exchange line to receive and make calls. 

Trunk-to-trunk transfer

A facility that allows an incoming call received on one line to be transferred to someone on another line, i.e. both callers are external to the telephone system. Typical uses include transferring an incoming caller to someone’s mobile phone. 

Unified messaging

A messaging platform that allows management of different electronic messages such as voice, email, SMS and video messages from a single user interface. Improves efficiency as all messages are presented in one place to the user. 

VLAN – Virtual Local Area Network

A protocol for separating devices on the same physical network. 

VoIP – Voice over Internet Protocol

Voice calls made over the Internet. 

VPN – Virtual Private Network

Provides users with secure inter-site communications over the Internet.  

WAN – Wide Area Network

A network that extends over a large geographical area connecting multiple Local Area Networks together. The Internet can be considered as the world’s largest WAN. A private WAN can be created to connect users in different sites to access the same information as if they were all connected locally on the one Local Area Network (see ‘ISP’ and ‘MPLS’). 

WAP – Wireless Access Protocol

A means of accessing the internet using a mobile phone. 


Wi-Fi is a WLAN (Wireless Local Area Network) technology. It provides short-range wireless high-speed data connections between mobile data devices (such as laptops, tablets or phones) and nearby Wi-Fi access points (special hardware connected to a wired network). 

WLR – Wholesale Line Rental

Offered the reseller the opportunity to take ownership of a BT line rental, allowing them to offer a one-bill solution and discounted pricing. Using the Openreach network, a customer could change their service provider seamlessly. The WLR service is being retired in December 2025, see our live blog for further information. 

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