UK VoIP & telecoms glossary

VoIP (Voice over Internet Protocol) is technology that carries telephone calls as data over an internet connection instead of over a traditional copper phone line.

A hosted PBX is a business telephone exchange that runs in a provider’s cloud rather than as hardware on your premises, delivering extensions, call routing, IVR and reporting as a subscription.

SIP (Session Initiation Protocol) is the signalling standard that sets up, modifies and ends voice and video calls over IP networks.

A SIP trunk is a virtual connection that links your own on-premise PBX to the public telephone network over the internet, replacing physical ISDN lines.

UCaaS (Unified Communications as a Service) is a cloud service combining voice, video, messaging and presence in one platform.

CCaaS (Contact Centre as a Service) is cloud software adding queues, dialers, wallboards and supervision for high-volume customer-contact teams.

An SBC (Session Border Controller) is a security and interoperability device that sits at the edge of a VoIP network controlling SIP signalling and media between domains.

SoGEA (Single Order Generic Ethernet Access) is a broadband product that delivers internet without a traditional phone line, designed for the post-PSTN world.

A codec is the algorithm that compresses and decompresses call audio; common VoIP codecs are G.711 (high quality), G.729 (low bandwidth) and Opus (adaptive).

Jitter is the variation in arrival time of voice data packets; high jitter causes choppy or robotic call audio.

MOS (Mean Opinion Score) is a 1–5 rating of perceived call quality, where 4.0+ is considered good business-grade voice.

SRTP (Secure Real-time Transport Protocol) encrypts the voice media stream of a call so it cannot be intercepted in transit.

A DID or DDI (Direct Inward Dialling) number is a dedicated external phone number that routes straight to a specific user, team or service without going through a switchboard.

Number porting is the regulator-protected process of moving an existing UK landline (via a STAC code) or mobile number (via a PAC code) to a new provider without changing the number.

CLI (Calling Line Identification) is the phone number presented to the person you call — your "caller ID".

A virtual landline is a UK phone number that works over the internet instead of a copper line, so it rings on an app, desk phone or mobile from anywhere — no physical line or line rental required.

The PSTN (Public Switched Telephone Network) is the traditional circuit-switched analogue telephone network that has carried UK voice calls for over a century.

The PSTN switch-off is the planned UK retirement of the analogue telephone network by BT Openreach, completing on 31 January 2027, after which all landlines must run over IP.

WLR2 (Wholesale Line Rental 2) is the BT Openreach wholesale product underneath nearly every analogue UK phone line, being withdrawn as part of the 2027 PSTN switch-off.

ISDN (Integrated Services Digital Network) is a legacy digital phone-line technology (ISDN2/ISDN30) used by larger UK sites for multi-channel voice, also retiring at the 2027 switch-off.

Emergency calling over VoIP is the ability to reach 999/112 (UK) or 911 (US) with an accurate registered address, which Ofcom requires VoIP providers to support.

IVR (Interactive Voice Response) is an automated phone menu that lets callers self-serve or route themselves to the right team using voice or keypad input.

ACD (Automatic Call Distribution) is the system that queues inbound calls and distributes them to agents by rules such as skill, priority or longest-idle.

A hunt group is a set of extensions that an inbound call rings in sequence or simultaneously until someone answers.

AMD (Answering Machine Detection) is technology that detects whether an outbound call was answered by a human or a voicemail so a dialer can react accordingly.

DTMF (Dual-Tone Multi-Frequency) is the tone signalling produced when you press phone keypad digits, used for menus and for entering card numbers by phone.

A softphone is a software application that turns a computer, browser or smartphone into a fully featured business phone.

Call forwarding (also called call divert) automatically sends calls to your number on to another destination — typically a mobile, colleague or voicemail — based on rules you set.

A call answering service makes sure inbound calls are answered — by a person or, increasingly, by an AI receptionist — when your team cannot pick up, capturing the caller, the reason and a callback.

An AI receptionist is software that answers your business phone in your brand voice, understands the caller, and resolves, books or routes the call 24/7.