What Is Business VoIP? A Complete Guide for UK Businesses
VoIP (Voice over Internet Protocol) enables business phone calls over an internet connection rather than traditional copper phone lines. With BT's PSTN switch-off deadline of January 2027, every UK business must migrate — this guide explains what business VoIP is, how it works, and what the options are.
Business VoIP: The Essentials
VoIP delivers phone calls over an internet connection, replacing traditional PSTN and ISDN lines. With BT's PSTN switch-off deadline of 31 January 2027, migration is mandatory for all UK businesses. 31% have already switched. Business VoIP typically costs 30–50% less than ISDN, works on any device from any location, and provides features that traditional phone systems cannot match.
Explore business VoIP optionsWhat Is VoIP and How Does It Work?
VoIP (Voice over Internet Protocol) converts spoken audio into compressed digital data and transmits it over an IP network — your broadband or leased line connection — to the recipient, where it is converted back into audio. The process is near-instantaneous. For the caller and recipient, a well-configured VoIP call is indistinguishable from a traditional phone call.
VoIP calls between two VoIP users travel entirely over the internet. Calls to traditional phone numbers — mobile numbers, landlines not yet migrated to VoIP — are converted at a gateway point from IP to the public telephone network. The caller does not need to know or care about this conversion; it happens transparently in the background.
SIP Trunking vs Hosted VoIP
There are two main ways to deploy business VoIP. SIP trunking replaces ISDN lines — your existing on-premise PBX phone system is kept, but the ISDN connection to the outside world is replaced with a SIP trunk (a virtual phone line over IP). This is cost-effective for businesses with a modern, compatible PBX that has useful remaining life.
Hosted VoIP (or UCaaS — Unified Communications as a Service) replaces the entire phone system with a cloud platform. There is no on-premise PBX at all; the provider manages all infrastructure. Staff access the system via desk phones, softphones on laptops, or mobile apps. Microsoft Teams Phone is the most common example for Microsoft 365 users. Hosted VoIP suits businesses with ageing PBX systems, those seeking the flexibility of mobile working, or those looking to simplify IT infrastructure.
Why UK Businesses Must Move to VoIP
BT Openreach is switching off the PSTN (Public Switched Telephone Network) and ISDN permanently on 31 January 2027. After that date, all analogue phone lines and ISDN connections will stop working. Every UK business currently on ISDN or analogue must migrate to an IP-based telephony solution before the deadline. There is no extension planned.
31% of UK businesses had already moved to VoIP as of 2025, with migration accelerating as the deadline approaches. Businesses that delay migration risk encountering constrained installer availability and number porting queues as demand increases in 2026.
Key Considerations for UK Businesses
- Assess your current phone system first: Understand what you have — ISDN2, ISDN30, analogue lines, or a mix — before deciding on a migration path. This determines whether SIP trunking or hosted VoIP is the better fit.
- Check your internet connection: VoIP calls require a stable internet connection with sufficient upload bandwidth. Each concurrent call needs approximately 80–100Kbps. Assess your current connection before migrating.
- Plan number porting well in advance: Your existing phone numbers can be kept, but porting takes 7–14 working days and must be coordinated with your new VoIP provider.
- Do not overlook non-telephony PSTN devices: Alarm systems, lift phones, PDQ terminals, and fax machines may also use PSTN lines that need to be migrated separately.
How AMVIA Can Help
AMVIA provides managed VoIP services for UK SMEs — from assessing your current phone system through to recommending the right solution, managing number porting, configuring the new system, and providing ongoing support. AMVIA has managed VoIP migrations for over 1,200 UK businesses. Call 0333 733 8050 to start your migration assessment today.
Business VoIP: Key Benefits
Why UK businesses switch from ISDN to VoIP.
Lower Cost
Typically 30–50% less than equivalent ISDN line rental with lower per-minute call rates.
Flexible Device Support
Make and receive calls on desk phone, laptop, or mobile — from any location with internet access.
Easy to Scale
Add or remove lines and users without engineer visits — changes take effect immediately.
Microsoft 365 Integration
Teams Phone integrates calling directly into Microsoft Teams — no separate phone system required.
Business VoIP Readiness Checklist
Steps to take before migrating to business VoIP.
Current phone system audited
All PSTN and ISDN lines identified — including alarms, lifts, and payment terminals.
VoIP migration path selected
Chosen between SIP trunking (keep PBX) or hosted VoIP / UCaaS (replace PBX).
Internet connection quality assessed
Upload speed and jitter checked for the number of concurrent calls at peak times.
Number porting planned
Existing numbers ported — 7–14 working days lead time confirmed with new provider.
Call divert configured
Inbound calls divert to mobiles if internet connection fails — continuity maintained.
Non-telephony PSTN services addressed
Alarm systems, lift phones, and payment terminals migrated separately before the deadline.
Business VoIP FAQs
On a well-configured connection, business VoIP call quality is equivalent to or better than a traditional ISDN call. Modern VoIP codecs provide high-definition voice quality that ISDN cannot match. Call quality issues — echo, delay, or dropped calls — are almost always caused by insufficient internet bandwidth, high jitter, or incorrect QoS configuration rather than VoIP itself. AMVIA assesses connectivity quality before migration and configures QoS as standard.
If your internet connection fails, VoIP calls to that location will not work. The standard mitigation is call divert — routing inbound calls to mobile numbers automatically when the internet is unavailable, so clients always reach someone. A backup 4G/5G router that activates automatically on broadband failure provides additional resilience. AMVIA configures both measures as part of a complete VoIP deployment.
Yes. Number porting transfers your existing geographic and non-geographic numbers to your new VoIP provider. The process takes 7–14 working days. During porting, calls continue on your existing lines until the port completes — there is no gap in service. AMVIA manages number porting as part of every VoIP migration it delivers.
Most businesses see a significant reduction in monthly telecoms costs after moving to VoIP — typically 30–50% compared to equivalent ISDN channel rental, before accounting for lower per-minute call rates. The saving depends on the number of ISDN channels being replaced and the call volumes involved. AMVIA provides a detailed cost comparison before any migration, so you can see the expected saving before committing.
Start Your VoIP Migration
AMVIA will assess your current phone system, compare VoIP options, and manage your migration — including number porting and configuration.
Related Resources
PSTN Switch-Off: What You Need to Do
The January 2027 deadline explained — who is affected and what to do.
What Is a SIP Trunk?
How SIP trunking works as an ISDN replacement for businesses with an existing PBX.
UCaaS for UK SMEs
When a fully hosted cloud phone system is the right VoIP migration path.
Business VoIP Solutions
AMVIA's managed VoIP services — supply, migration, and ongoing management.