What Is Business VoIP? A Complete Guide for UK Businesses

What Is VoIP and Why Does It Matter for UK Businesses?

As part of our comprehensive business VoIP guide, this page explains what Voice over Internet Protocol is, how it works at a technical level, and why every UK organisation must now take it seriously. VoIP (Voice over Internet Protocol) is the technology that converts spoken audio into compressed digital data packets and transmits them over an IP network — typically your business broadband or leased line connection — to the recipient, where they are reassembled and converted back into audible speech. The entire process takes milliseconds. On a properly configured connection, a VoIP call is indistinguishable from a traditional landline call in terms of clarity and reliability.

The reason VoIP matters now more than ever is straightforward: the UK PSTN switch-off is scheduled for completion by January 2027 (Openreach/BT). After that date, every analogue phone line and ISDN connection in the country will cease to function. Over 2 million UK businesses still rely on PSTN/ISDN lines (Ofcom estimates), and each one must migrate to an IP-based telephony solution before the deadline arrives. There is no planned extension, and Openreach has already stopped accepting new PSTN line orders in the majority of exchange areas.

How VoIP Works: The Technical Fundamentals

When you speak into a VoIP-enabled device — whether that is a desk phone, a softphone application on your laptop, or a mobile app on your smartphone — the analogue audio from your voice is digitised by a codec. A codec (coder-decoder) compresses the audio into small data packets, each tagged with routing information so the network knows where to send them. These packets travel across your internet connection using protocols such as SIP (Session Initiation Protocol) for call setup and RTP (Real-time Transport Protocol) for media delivery.

At the receiving end, the packets are reassembled in the correct order and decoded back into audio. Modern codecs such as G.722 and Opus deliver high-definition voice quality that actually surpasses the audio fidelity of traditional ISDN calls, which were limited to a narrowband frequency range. The result is clearer, richer-sounding calls — provided the underlying internet connection meets the necessary quality thresholds.

VoIP calls between two VoIP users travel entirely over the internet. Calls to traditional phone numbers — mobile numbers or landlines not yet migrated — are converted at a gateway point from IP to the public switched telephone network. The caller does not need to know or care about this conversion; it happens transparently in the background.

Bandwidth and Quality of Service Requirements

Each concurrent VoIP call requires approximately 80–100 Kbps of both upload and download bandwidth when using the G.711 codec, or around 30–40 Kbps with more efficient codecs such as G.729. For a business with 20 staff making simultaneous calls at peak times, that equates to roughly 2 Mbps of dedicated upload bandwidth — well within the capacity of most modern FTTC or FTTP broadband connections.

However, bandwidth alone does not guarantee call quality. Three additional metrics matter: latency (the delay between speaking and being heard), jitter (variation in packet arrival times), and packet loss (the percentage of data packets that fail to arrive). For acceptable call quality, latency should remain below 150 ms, jitter below 30 ms, and packet loss below 1%. Quality of Service (QoS) settings on your router can prioritise voice traffic over bulk data transfers, ensuring calls remain clear even when the connection is under load from other applications.

SIP Trunking vs Hosted VoIP: Two Migration Paths

There are two principal ways to deploy business VoIP, and choosing between them depends on your existing infrastructure and long-term plans.

SIP trunking replaces your ISDN lines whilst keeping your existing on-premise PBX phone system in place. The ISDN connection to the outside world is replaced with a SIP trunk — a virtual phone line delivered over your internet connection. This is the most cost-effective route for businesses with a modern, SIP-compatible PBX that has several years of useful life remaining. The PBX continues to manage internal call routing, hunt groups, and extensions exactly as before; only the external connection changes.

Hosted VoIP (also known as UCaaS — Unified Communications as a Service) replaces the entire phone system with a cloud-hosted platform. There is no on-premise PBX hardware at all; the provider manages all infrastructure in their data centres. Staff access the system via IP desk phones, softphone applications on laptops, or mobile apps. Microsoft Teams Phone is the most widely adopted example for organisations already using Microsoft 365 — with Microsoft Teams now having over 320 million monthly active users globally (Microsoft 2024). Hosted VoIP suits businesses with ageing PBX systems approaching end of life, those embracing hybrid working, or those seeking to consolidate their IT infrastructure.

Core Features of a Business VoIP System

Business VoIP systems — whether delivered via SIP trunking or hosted UCaaS — provide a range of features that traditional analogue and ISDN systems either cannot offer or require expensive add-on hardware to support.

• Auto-attendant and IVR: Automated greeting and menu systems that route callers to the correct department or individual without a human receptionist. Configured through a web portal rather than requiring an engineer.
• Hunt groups and call queues: Incoming calls distributed across a group of extensions based on rules you define — ring all simultaneously, round-robin, longest idle, or skills-based routing.
• Call recording: Automatic or on-demand recording of calls for training, compliance, or dispute resolution. Recordings stored in the cloud and searchable by date, caller, or extension.
• Voicemail to email: Voicemail messages delivered as audio attachments to your email inbox, with transcription available on many platforms so you can read the message without listening to it.
• Presence and status: Real-time visibility of whether colleagues are available, on a call, or away — reducing internal call attempts to engaged or absent staff.
• Mobile and desktop apps: Make and receive calls on your business number from any device, anywhere with an internet connection. Your business number is displayed to the recipient regardless of which device you use.
• CRM integration: Caller information from your CRM system displayed on-screen when a call arrives, enabling staff to greet callers by name and access their account history immediately.

Benefits of Moving to Business VoIP

The financial case for VoIP is compelling. Most UK businesses see a 30–50% reduction in monthly telephony costs after migrating from ISDN, driven primarily by the elimination of ISDN line rental charges (typically £25–£50 per channel per month) and lower per-minute call rates. For a business with 30 ISDN channels, this can represent savings of several hundred pounds per month.

Beyond cost, VoIP delivers operational benefits that traditional telephony cannot match. Scalability is immediate — adding a new user or an additional call channel is a software change that takes effect within minutes, with no engineer visit required. Geographic flexibility means staff can work from any location with an internet connection and still make and receive calls on their business number, which is essential for the hybrid working patterns now standard across UK businesses. Integration with platforms such as Microsoft 365 and popular CRM systems streamlines workflows and reduces the friction of switching between applications.

Disaster recovery is also stronger with VoIP. If a physical office becomes inaccessible — whether due to a power cut, flooding, or any other disruption — calls can be automatically diverted to mobile devices or alternative locations. With a traditional ISDN system, losing the office means losing your phone lines entirely until the premises are accessible again.

Why UK Businesses Must Act Before January 2027

The UK PSTN switch-off is scheduled for completion by January 2027 (Openreach/BT), and there is no indication of any extension. After that date, all analogue phone lines and ISDN connections will permanently cease to function. This is not a gradual phase-out — it is a hard cut-off. Openreach has already stopped selling new PSTN and ISDN products in hundreds of exchange areas across the country, and the stop-sell programme is expanding continuously.

Over 2 million UK businesses still rely on PSTN/ISDN lines (Ofcom estimates), and migration activity is accelerating sharply. Businesses that delay risk encountering constrained installer availability, longer number porting queues, and reduced choice of migration slots as demand peaks through 2026. The practical advice is clear: begin your migration assessment now, even if your planned go-live date is months away, to secure adequate lead time for number porting (typically 7–14 working days) and system configuration.

It is also critical to remember that the PSTN switch-off does not only affect desk phones. Alarm systems, lift emergency telephones, PDQ card payment terminals, fax machines, door entry systems, and CCTV monitoring connections that use PSTN lines will all stop working on the same date. Each of these requires a separate migration plan — often involving IP or 4G alternatives — and should be audited as part of your overall switch-off preparation.

Key Considerations Before Migrating

• Audit your current phone system thoroughly: Identify every PSTN and ISDN line your business uses — including those connected to non-telephony devices such as alarms, lifts, and payment terminals. This determines whether SIP trunking or hosted VoIP is the better fit and ensures nothing is overlooked.
• Assess your internet connection quality: VoIP calls require a stable connection with sufficient upload bandwidth and low jitter. Have your connection tested under real-world load conditions before committing to a migration date.
• Plan number porting well in advance: Your existing phone numbers can almost always be kept, but porting takes 7–14 working days and must be coordinated carefully with your new provider to avoid any gap in service.
• Consider your connectivity type: If your broadband is delivered over FTTC (which uses the copper PSTN network for the final section), you may need to upgrade to FTTP or a dedicated leased line to ensure your internet connection itself is not affected by the switch-off.
• Evaluate Microsoft Teams integration: If your business already uses Microsoft 365, Microsoft Teams Calling may be the most natural VoIP migration path — consolidating your phone system into the platform your staff already use daily.

How AMVIA Can Help

AMVIA provides fully managed VoIP services for UK SMEs — from initial assessment of your current phone system through to recommending the right solution, managing number porting, configuring the new system, and providing ongoing support. Whether SIP trunking, hosted UCaaS, or Microsoft Teams Phone is the right fit, AMVIA will assess your requirements honestly and manage the migration end to end. AMVIA has delivered VoIP migrations for over 1,200 UK businesses. Call 0333 733 8050 to start your migration assessment today.