Telecoms glossary

A2P (application-to-person) SMS is a text message sent automatically by software to a mobile user, such as a one-time passcode, delivery update, appointment reminder or marketing offer, through a messaging platform connected to mobile networks.

ACD (automatic call distribution) is the phone system feature that hands calls waiting in a queue to available agents according to a rule you choose, such as round robin, longest idle or skills-based routing.

BYOC (bring your own carrier) means connecting a communications platform, such as Microsoft Teams, a cloud contact centre or a voice-agent platform, to the carrier of your choice over a SIP trunk, instead of using the telephony the platform sells.

Call forwarding automatically redirects calls arriving on a number or extension to another destination, such as a mobile, a colleague or voicemail, either always or only when the line is busy or unanswered.

A call queue holds incoming callers when every agent is busy and connects them in order of arrival as soon as someone is free, playing music and announcements while they wait.

Call recording saves business phone conversations as audio files for training, evidence, compliance or quality checks. Callers must be told, and the recordings are personal data under GDPR.

Call termination is the final leg of a phone call: carrying it across the called person's network, which charges the originating carrier a termination fee. The cost depends on the country, the line type (landline or mobile) and the route used.

A call transfer passes a call that has already been answered to another extension or number, either straight away (blind transfer) or after briefing the colleague first (warm or attended transfer).

Caller ID, or CLI (calling line identification), is the caller's number passed through the network and shown to the person called. A business may only present a valid number that it is entitled to use.

CCaaS (contact centre as a service) is cloud-hosted contact centre software. It receives calls, emails and messages, routes them to agents, manages queues and gives supervisors live dashboards and reports, on a per-agent subscription.

A cloud PBX is a business phone system run by a provider in its data centres. Extensions, IVR menus, queues and call routing are managed from a web portal, with no PBX hardware on site.

A codec is the algorithm that digitises and compresses your voice during a VoIP call and turns it back into sound at the other end. It sets both the audio quality and the bandwidth used.

A contact centre is the team, processes and software that handle customer conversations across several channels, such as phone, email, chat and messaging, using shared queues, agents and reporting.

CPaaS (communications platform as a service) is a cloud service that provides communication building blocks, such as voice calls, SMS, WhatsApp, video and phone numbers, as APIs that developers embed in their own applications.

CRM integration connects a business phone system to customer relationship management software, so the caller's record opens when they ring, staff dial with one click and every call is logged automatically.

CTI (computer telephony integration) connects the phone system to business software, so the caller's record appears when the phone rings and you can dial a number with one click.

A DID (direct inward dialling) number, called a DDI in the UK, is a public phone number that rings a specific extension, team or call flow directly, without passing through a switchboard.

DTMF (dual-tone multi-frequency) is the signal a phone sends when you press a key during a call: two tones played together for each key, which an IVR or remote system decodes.

An eSIM is a SIM chip built into a phone or device. Instead of inserting a plastic card, the carrier downloads a subscription profile to it remotely, usually by scanning a QR code.

An extension is a short internal number, usually three or four digits, given to a user or device on a business phone system. Colleagues dial it to reach each other, and the system uses it to route calls.

A freephone number is a number that is free for the caller, such as 0800 and 0808 in the UK or 1800 in Ireland. The business pays for the call, so it suits helplines and campaigns where cost could put callers off.

GDPR applies to business telephony whenever it handles personal data: phone numbers, call logs, recordings and voicemails. It requires a defined purpose and lawful basis, clear information for callers and staff, limited retention and safeguards from suppliers.

A geographic number is a landline number whose area code points to a place, such as 020 for London or 01 for Dublin. It signals a local presence and is charged as a standard national call.

IVR (interactive voice response) is the automated phone menu that greets callers and routes them based on the keys they press, such as “press 1 for sales, press 2 for support”.

Jitter is the variation in the arrival time of voice packets. When it exceeds what the phone's jitter buffer can absorb, packets are discarded and the voice sounds choppy or robotic.

Latency is the time your voice takes to travel from your mouth to the other person's ear. In VoIP, once one-way delay passes about 150 ms, conversations start to feel stilted.

A missed call is an inbound call that nobody answered: the caller hung up while it rang or while waiting in a queue, or reached voicemail without speaking to anyone.

MOS (mean opinion score) rates perceived call quality on a scale of 1 to 5. In VoIP it is estimated from latency, jitter, packet loss and the codec in use, without anyone listening.

An MVNO (mobile virtual network operator) sells mobile lines and plans without owning radio infrastructure. It rents capacity from a host network such as EE, Vodafone, O2 or Three and runs its own customers, tariffs and billing.

A non-geographic number is a number not linked to any area, such as UK 03, 084 or 087 numbers. 03 numbers cost the same as a local call; 084 and 087 add a service charge set by the business.

Number porting is moving an existing phone number from one provider to another so the business keeps the same number. The new provider runs the process; the old service ends when the port completes.

A PBX (private branch exchange) is a business's own telephone switch. It connects internal extensions to each other and to the public network, and runs features such as IVR menus, queues and voicemail.

QoS (quality of service) is the set of network settings that give time-sensitive traffic, such as voice, priority over other data, so calls avoid delay, jitter and packet loss when the link is busy.

A ring group (or hunt group) rings several extensions for the same number, either all at once or one after another, so the first available colleague can pick up the call.

Roaming lets you use your mobile line abroad on a partner operator's network. Within the EU and EEA, roam like at home rules apply domestic prices; UK networks have set their own charges since Brexit.

A session border controller (SBC) sits at the edge of a network and controls SIP calls going in and out: it filters traffic, hides the internal network, handles NAT and can transcode or encrypt calls.

SIP (Session Initiation Protocol) is the standard signalling protocol that sets up, changes and ends VoIP calls. It handles ringing, answering and hanging up; the voice itself travels separately.

A SIP trunk is a voice-over-IP connection between a business phone system and a carrier. It replaces ISDN lines and carries calls and numbers over your internet connection.

A softphone is a phone application that runs on a computer or smartphone. It replaces the desk phone and connects to the company phone system over VoIP, using the business number.

STIR/SHAKEN is a set of standards that lets the originating carrier digitally sign a call to vouch that the caller is entitled to the number shown, so terminating carriers can detect spoofed caller ID.

A telecom API is a programming interface that lets software control communication services: place or receive calls, manage phone numbers, send SMS, pull call history or get notified in real time when something happens on a call.

The Telephone Preference Service (TPS) is the UK's free opt-out register for unsolicited live sales and marketing calls. Under PECR, businesses must not make such calls to registered numbers unless the person has told that caller they are happy to be called.

UCaaS (unified communications as a service) brings business telephony, video meetings, instant messaging and often file sharing together in one cloud service, billed per user and used from desk phones, computers or mobiles.

A virtual phone number is a real phone number with no physical line behind it: calls are routed over the internet to a mobile app, a softphone or a phone system, wherever you are.

Voicemail records a message when a call goes unanswered and makes it available to the recipient on their phone, in an app or by email, often with the caller's number and the time.

VoIP (voice over internet protocol) carries phone calls as packets of data over an IP network, such as the internet or an office LAN, instead of over the copper lines of the traditional phone network.

A webphone is a business phone that runs in a browser tab with nothing to install. It uses WebRTC to make and receive calls through the company phone system on the business number.

WebRTC is an open standard built into web browsers that lets people make voice and video calls from a web page with no plug-in or app to install, with encryption always switched on.

The WhatsApp Business API is Meta's business access to WhatsApp. It connects a company number to software such as a CRM, helpdesk or phone system, so several people can answer customers and messages sit alongside other channels.