What Is Business VoIP? A Complete Guide for UK Businesses

What is business VoIP, and why does it matter now?

VoIP converts your voice into digital data packets and sends them over an IP network — your broadband or leased line — then reassembles them into speech at the other end in milliseconds. On a properly configured connection, a VoIP call is indistinguishable from a landline call. It is the modern replacement for analogue and ISDN telephony, and the core of every business VoIP deployment.

The timing matters because the UK's PSTN — the old analogue phone network — is being switched off, and Ofcom has confirmed all voice services must move to digital, internet-based calling (Ofcom). The industry completion date is 31 January 2027. After that, analogue lines and ISDN connections stop working — a hard cut-off, not a gradual phase-out.

More than 2 million UK businesses still rely on PSTN/ISDN lines (Ofcom estimates), and Openreach has already stopped selling new PSTN and ISDN products across hundreds of exchange areas. Roughly 31% of businesses have already switched; the rest are now on the clock.

How does VoIP actually work?

When you speak into a desk phone, softphone or mobile app, a codec digitises and compresses your voice into small packets tagged with routing information. Those packets travel across your internet connection using SIP (Session Initiation Protocol) to set up the call and RTP (Real-time Transport Protocol) to carry the audio, then are reassembled and decoded at the far end.

Modern codecs such as G.722 and Opus deliver high-definition voice that surpasses the narrowband audio of traditional ISDN. Calls between two VoIP users stay entirely on the internet; calls to mobiles or not-yet-migrated landlines convert from IP to the public network at a gateway, transparently in the background.

How much bandwidth does VoIP need?

Each concurrent call needs roughly 80–100 Kbps up and down on the G.711 codec, or 30–40 Kbps on more efficient codecs like G.729. A 20-person office at peak load needs around 2 Mbps of dedicated upload — comfortable for most FTTP or FTTC connections.

Bandwidth alone is not enough. Three quality metrics decide whether calls sound clean:

• Latency — keep below 150 ms (the delay between speaking and being heard).
• Jitter — keep below 30 ms (variation in packet arrival times).
• Packet loss — keep below 1% (packets that never arrive).

Quality of Service (QoS) settings on your router prioritise voice over bulk data, so calls stay clear even when the line is busy.

SIP trunking vs hosted VoIP — which path is right?

There are two main ways to deploy business VoIP. SIP trunking replaces your ISDN lines but keeps your on-premise PBX; hosted VoIP (UCaaS) replaces the phone system entirely with a cloud platform. The right choice depends on the age and life left in your current PBX.

SIP trunking is the most cost-effective route when you have a capable PBX — only the external connection changes. SIP trunking keeps hunt groups, extensions and call routing exactly as they are.

Hosted UCaaS for SMEs suits businesses with end-of-life hardware or hybrid working. For organisations already on Microsoft 365, Microsoft Teams Calling is the most natural path — Teams reports over 320 million monthly active users globally (Microsoft, 2024), so staff are already in the app (Microsoft 365).

What features does a business VoIP system include?

Business VoIP — whether SIP or hosted — provides features that analogue and ISDN systems either cannot offer or need expensive add-on hardware to support. Most are configured through a web portal, not an engineer visit.

• Auto-attendant and IVR — automated greeting and menus that route callers to the right department.
• Hunt groups and call queues — distribute inbound calls by ring-all, round-robin, longest-idle or skills-based rules.
• Call recording — automatic or on-demand, stored in the cloud, searchable by date, caller or extension.
• Voicemail to email — messages delivered as audio attachments, with transcription on many platforms.
• Presence and status — see who is available, on a call or away before you dial.
• Mobile and desktop apps — make and take calls on your business number from any device, anywhere.
• CRM integration — caller account history on-screen the moment a call lands.

What are the benefits of moving to business VoIP?

The financial case is strong: many UK businesses cut monthly telephony costs by around 30–50% (typical UK saving, 2026) after leaving ISDN, mainly by removing ISDN line rental and lowering per-minute rates. But the operational gains matter just as much.

Scaling is immediate — adding a user or channel is a software change that takes effect in minutes. Geographic flexibility means staff make and receive calls on their business number from any location, which is essential for hybrid working. Integration with Microsoft 365 and CRM systems removes friction between apps.

Resilience improves too. If an office becomes inaccessible — power cut, flooding, anything — calls divert automatically to mobiles or another site. With ISDN, losing the building means losing the phone lines until you are back in.

Why must UK businesses act before the PSTN switch-off?

The switch-off is a hard deadline, not a soft target. Once the analogue network is retired, every PSTN line and ISDN connection stops permanently, and Openreach's stop-sell programme is already live across hundreds of exchanges. Acting early protects your install slot and your numbers.

The risk of delay is practical: constrained installer availability, longer number-porting queues and fewer migration slots as demand peaks. Start your assessment now even if go-live is months out — number porting alone takes 7–14 working days. Read the full PSTN switch-off breakdown for the affected-services detail.

It is not only desk phones. Alarm systems, lift emergency lines, PDQ card terminals, fax machines, door entry and PSTN-connected CCTV all stop on the same date and each needs its own IP or 4G/5G migration plan.

What should you check before migrating?

Audit every PSTN and ISDN line first — including non-telephony devices like alarms, lifts and payment terminals — so nothing is missed and you can choose SIP or hosted correctly. Then test connection quality under real load, plan number porting early, and decide whether your access network itself needs upgrading.

• Connectivity type — if your broadband runs over FTTC (copper for the final hop), consider FTTP or a dedicated leased line so the access network itself is switch-off-proof.
• Call continuity — configure inbound divert to mobiles, plus a 4G/5G backup router that activates on broadband failure.
• Security — VoIP is an internet service and needs hardening against toll fraud and eavesdropping; see VoIP security and apply the NCSC's guidance on protecting voice services (NCSC).

How AMVIA helps you migrate

AMVIA provides fully managed VoIP for UK SMEs — from auditing your current phone system and recommending the right fit, through number porting, configuration and ongoing support. Whether SIP trunking, hosted UCaaS or Microsoft Teams Phone is right, AMVIA assesses your needs honestly and manages the migration end to end. AMVIA has delivered VoIP migrations for over 1,200 UK businesses. Call 0333 733 8050 to start your assessment.