VoIP Phone Line for Business | How It Works & Costs

The AMVIA Team | 5 min read · Mar 2026

With the copper network being switched off, every UK business will move its phone lines to IP in the next few years. If you run a 10–500 person organisation, understanding how a VoIP phone line actually works — and what it should cost — is the difference between a clean migration and an expensive scramble. This guide explains the line types, the numbers, and the practical setup, written by people who run business VoIP deployments for a living.

What is a VoIP phone line?

A VoIP phone line is the internet-based equivalent of a traditional telephone line. Instead of a physical copper wire running from the exchange to your premises, the line is a virtual connection carried over your broadband. Calls are converted to digital data, transmitted over the internet, and reassembled at the far end within milliseconds.

For a business, a VoIP phone line rarely exists on its own. It sits inside a hosted phone system (cloud PBX), where it is represented as either a hosted extension or a SIP trunk channel. Each user or active call path consumes one line while a call is live. If you want the full picture of how the system around the line works, see our hosted phone system breakdown.

How does a VoIP phone line differ from a traditional line?

A VoIP phone line is defined in software, priced per user, and works anywhere there is internet. A traditional PSTN or ISDN line is a fixed physical circuit, priced per line, and tied to one building. That single shift — from hardware to software — is what makes VoIP cheaper, faster to scale, and location-independent.

The contrast is sharpest across three areas:

Physical infrastructure

A traditional line needs a copper connection from the telephone exchange to your building. Capacity is fixed: 10 ISDN channels means 10 simultaneous calls, and adding more means ordering lines and waiting for an engineer. A VoIP phone line is software — capacity is set by your bandwidth and your plan, and adding lines is usually an online configuration change.

Cost structure

Traditional lines carry a fixed rental regardless of how much you call. VoIP is priced per user (extension), and UK call charges are typically bundled into the monthly fee. For most business scenarios that makes VoIP structurally cheaper, with one predictable per-seat cost rather than line rental plus metered calls.

Geographic flexibility

A traditional line is physically tied to a location. A VoIP line follows the user. Your business number — a London 020, a Birmingham 0121, or a national 03 — rings on the same extension whether the user is in the office, at home, or on the road.

What are the types of VoIP phone line?

There are three common forms of VoIP phone line for UK businesses: the hosted extension, the SIP trunk, and Teams direct routing. Which one fits depends on whether you want a full cloud system, want to keep an existing on-premise PBX, or run everything through Microsoft Teams.

Hosted extension

A hosted extension is a user-based VoIP line inside a cloud phone system. Each user gets an extension number and takes calls through an IP desk phone, a desktop softphone, or a mobile app. This is the most common form of VoIP line for business users and the simplest to manage.

SIP trunk

A SIP trunk connects an on-premise PBX to the carrier network over the internet, replacing ISDN30 lines while keeping your existing PBX hardware. It is priced per channel (concurrent call). If you have recently invested in a PBX and want to retain it, our SIP trunk service is the bridge to IP.

Cloud direct routing

Direct routing connects a Microsoft Teams environment to a carrier's SIP infrastructure, letting Teams make and receive calls to standard phone numbers. It suits Teams-centric organisations that want calling inside the tool staff already use — covered in detail on our Microsoft Teams direct routing page.

What does a VoIP phone line cost?

VoIP phone line costs depend on the product type, but most UK SMEs land on a hosted extension at a single per-user fee that includes calls. SIP trunks are cheaper per channel but exclude call charges and assume you already run a PBX. Numbers are a small add-on, often included.

• Hosted extension (per user): from £5.95 per user per month, including UK calls
• SIP trunk (per channel): £4–£8 per channel per month, plus call charges (typical UK 2026 range)
• Geographic phone number: £1–£5 per month per number (often included)
• Non-geographic numbers (0800, 03): £3–£10 per month per number

For most SMEs the hosted extension wins: it bundles calls and features into one per-user fee with no on-premise PBX to maintain. SIP trunking only beats it when you have a recent PBX investment to protect.

The wider market is moving the same way. "Europe's SIP trunking market is growing at a projected CAGR of 9.8% over the forecast period, driven by PSTN retirements and demand for cost-efficient communication solutions." (Zionmarketresearch). The driver is the copper switch-off, which Openreach is rolling out across the UK as all-IP migration.

How do you get a VoIP phone number?

When you set up a VoIP phone line you can either pick a fresh number from any UK area code or port your existing BT or other carrier numbers across. Porting keeps every published and printed number working, so customers notice nothing while the underlying line moves to IP.

Number porting transfers your existing numbers — 01, 02, 03, 0800 and more — from any UK carrier to your VoIP provider. The process typically takes 5–10 working days with no disruption to incoming calls. AMVIA selects the right numbers and manages the porting end to end, so you keep continuity while we handle the carrier paperwork.

Is a VoIP phone line reliable and secure enough for business?

Yes. Modern VoIP from a reputable provider runs on high-availability platforms with strong uptime, and call quality is governed mainly by your internet connection. A business-grade broadband or leased line connection gives consistent, clear calls, and failover routing keeps you reachable during an outage.

Reliability is now a point in VoIP's favour, not against it. "PSTN resilience incidents increased 45% in 2024 — and 2024/25 saw over 2,600 major PSTN resilience incidents — making legacy lines demonstrably less reliable than modern VoIP infrastructure." (UK Government). The ageing copper network is being retired precisely because it is harder to keep stable.

Security matters because a VoIP line is, ultimately, traffic on your network. Toll fraud, eavesdropping, and compromised handsets are real risks, and the National Cyber Security Centre publishes guidance on securing internet-connected communications. AMVIA hardens every deployment — covered on our VoIP security page — because we are a security partner first. One provider. Security-first. Microsoft-certified.