How to Migrate from a Traditional PBX to VoIP

Every year, thousands of UK businesses face the same inflection point: their ageing PBX system is becoming expensive to maintain, spare parts are scarce, and the supplier who installed it a decade ago has long since been acquired or shuttered. With the BT/Openreach PSTN switch-off confirmed for January 2027, the question is no longer whether to migrate to VoIP — it’s how to do it without disrupting the phones your staff and customers depend on every single day.

This guide walks you through the entire migration journey, from the initial infrastructure audit right through to post-cutover support. Whether you run a 10-handset solicitors’ practice or a 500-seat contact centre, the principles are the same — and getting them right is the difference between a seamless transition and a week of missed calls, frustrated staff, and lost revenue.

Step 1: Assess Your Current PBX Infrastructure

Before you can plan a migration, you need a brutally honest picture of what you already have. Many businesses are surprised by the complexity lurking inside their comms cupboard — trunks they forgot about, hunt groups configured years ago by a departed engineer, or analogue lines feeding fax machines, door entry systems, and lift phones that nobody thought to document.

The Hardware Audit

Start by cataloguing every piece of equipment connected to your existing PBX. This includes the central unit itself (manufacturer, model, firmware version, age), all handsets and their locations, any expansion cards (ISDN BRI/PRI, analogue trunk cards, voicemail modules), conference phones, DECT bases and cordless handsets, and analogue devices such as fax machines, franking machines, card payment terminals, and alarm diallers.

Call Flow & Feature Mapping

Document every call route, auto-attendant menu, ring group, hunt group, call queue, voicemail box, and speed dial. You need to understand exactly how calls flow through the organisation today so you can replicate — and ideally improve — those flows in the new VoIP platform. Pay particular attention to:

• DDI (Direct Dial-In) numbers — how many do you have, and which departments or individuals do they route to?
• Call recording — is it active? Where are recordings stored? Are there FCA or MiFID II compliance requirements?
• IVR menus — map out every menu tree, including out-of-hours and bank holiday schedules.
• CRM integrations — does your PBX feed call data into Salesforce, HubSpot, or a bespoke system?
• Disaster recovery — what happens today if the PBX fails? Is there a failover route?

Step 2: Evaluate Your Network Readiness

VoIP is only as good as the network it runs on. A traditional PBX uses dedicated copper pairs for each call — bandwidth was never a concern. With VoIP, every call is a data stream competing for bandwidth alongside email, web browsing, cloud applications, and file transfers. If your network isn’t ready, call quality will suffer.

Bandwidth Requirements

A single VoIP call using the G.711 codec requires approximately 85–100 Kbps in each direction. If your busiest period sees 30 concurrent calls, you need around 3 Mbps of dedicated, uncontested bandwidth just for voice. That sounds modest, but the key word is uncontested — a shared 100 Mbps FTTP line that’s saturated by a large file upload will still produce choppy calls.

Quality of Service (QoS)

Your switches, routers, and firewall must support QoS tagging to prioritise voice packets (typically DSCP EF – Expedited Forwarding) over other traffic. Without QoS, a colleague downloading a 2 GB file will cause jitter and packet loss on active calls. If your network kit is older than five or six years, it may lack the granular QoS controls modern VoIP demands.

Network Readiness Checklist

Requirement	Minimum Standard	Recommended
Internet bandwidth	100 Kbps per concurrent call	Dedicated SIP trunk or SD-WAN
Latency	< 150 ms one-way	< 80 ms one-way
Jitter	< 30 ms	< 15 ms
Packet loss	< 1%	< 0.1%
QoS support	DSCP tagging at switch level	End-to-end QoS with VLAN segregation
Power over Ethernet (PoE)	PoE switches or injectors for IP phones	PoE+ switches with UPS backup
Firewall	SIP ALG disabled, required ports open	SBC (Session Border Controller) deployed

Step 3: Choose Your Migration Strategy

There are three established approaches to migrating from PBX to VoIP. Each carries different levels of risk, cost, and complexity. The right choice depends on your organisation’s size, risk tolerance, and the criticality of uninterrupted phone service.

Migration Strategy Comparison

Factor	Phased Migration	Big Bang Cutover	Parallel Running
Duration	4–12 weeks	1–2 weekends	6–16 weeks
Risk level	Low–Medium	High	Very Low
Cost	Moderate	Lowest	Highest (dual running costs)
Downtime	Minutes per department	Hours (planned weekend)	Near zero
Rollback complexity	Simple (per department)	Very difficult	Instant (switch back to PBX)
Best suited for	20–500 users	< 20 users	Contact centres & regulated industries
Staff training approach	Train each group before their cutover	All staff trained in advance	Train while both systems live

For the vast majority of UK SMEs, Cloudswitched recommends a phased migration. It balances speed with safety, allows you to learn from each phase before moving to the next, and keeps disruption contained to small groups at a time.

Step 4: Number Porting — The Ofcom Process

Your phone numbers are business-critical assets. Customers, marketing materials, Google Business Profile, directory listings — they all reference your existing numbers. Losing them is not an option. Fortunately, Ofcom’s General Conditions guarantee your right to port numbers between providers in the UK.

How Number Porting Works

The process is governed by Ofcom’s number portability regulations. Your new VoIP provider (the “gaining provider”) submits a porting request to your current provider (the “losing provider”). The losing provider has one business day to validate the request. Once validated, the port is scheduled — typically within 5–10 working days for geographic numbers.

Key things to know:

• Geographic numbers (01/02 prefix) port within 5–10 working days after validation.
• Non-geographic numbers (03, 0800, 0845, etc.) can take longer — up to 15 working days.
• You cannot port numbers that are subject to an active dispute or debt with the losing provider.
• Port dates are agreed in advance — you choose the date and time. Most businesses opt for early morning or a weekend to minimise impact.
• There should be minimal downtime during the port itself — typically seconds to a few minutes. However, it’s wise to set up temporary call forwarding as a safety net.

Step 5: Plan Your Project Timeline

A well-structured migration project has clearly defined phases, each with measurable deliverables. Here is a typical timeline for a phased migration of a 50–100 user organisation:

Phase	Duration	Key Activities	Deliverable
1. Discovery & Audit	Week 1–2	Hardware audit, call flow mapping, network assessment, stakeholder interviews	Migration readiness report
2. Design & Planning	Week 2–3	VoIP platform selection, call flow design, number porting paperwork, hardware ordering	Signed-off migration plan
3. Network Preparation	Week 3–4	Switch/firewall upgrades, VLAN configuration, QoS implementation, PoE provisioning	Network readiness sign-off
4. Platform Build	Week 4–6	VoIP system configuration, IVR menus, ring groups, call queues, integrations, voicemail	Configured platform (staging)
5. Pilot Phase	Week 6–7	Migrate pilot group (5–10 users), monitor call quality, gather feedback	Pilot sign-off & lessons learned
6. Phased Rollout	Week 7–10	Migrate remaining departments in waves, number porting, handset deployment	All users on VoIP
7. Decommission & Support	Week 10–12	Decommission PBX, cancel legacy lines, hand over to support, documentation	Project closure report

Step 6: Budget & Cost Breakdown

One of the strongest drivers for VoIP migration is cost. Legacy PBX systems carry significant ongoing costs: ISDN line rental (£200–£400 per month for a PRI), maintenance contracts (£1,000–£5,000 per year), and expensive engineer call-outs when something breaks. VoIP consolidates everything into a predictable per-user monthly fee.

Here is a typical cost breakdown for a 50-user phased migration:

For a 50-user deployment, total one-off migration costs typically fall between £8,000 and £20,000 depending on whether you need new network infrastructure. The ongoing monthly cost for hosted VoIP usually lands between £8 and £18 per user per month — significantly less than the combined cost of ISDN lines, PBX maintenance, and call charges on a traditional system.

Step 7: Staff Training & Change Management

Technology migrations fail when people are left behind. Your staff have been using the same desk phones — possibly the same handsets — for years. They know which button transfers a call, how to set up a conference, and where to check their voicemail. Changing all of that without proper training breeds frustration, support tickets, and a general sense that the new system is “worse than the old one.”

Training Best Practices

• Train in small groups (8–12 people) by department, not in a single all-hands session. Different teams use the phone system differently.
• Provide quick-reference cards at every desk — laminated A5 sheets covering the five most common tasks: make a call, transfer a call, set up a conference, check voicemail, set Do Not Disturb.
• Identify champions in each department — tech-savvy individuals who receive advanced training and act as first-line support for their colleagues.
• Schedule training 2–3 days before cutover, not 2–3 weeks. Skills fade fast if there’s a gap between learning and doing.
• Record a 10-minute video walkthrough that staff can refer back to at any time.

Step 8: Testing & Go-Live Checklist

Never go live without a structured testing plan. At Cloudswitched, we run through a comprehensive test script for every migration that covers inbound calls to every DDI, outbound calls to landlines, mobiles, and international numbers, call transfers (both blind and attended), ring groups and hunt group failover, IVR menu navigation, voicemail recording, playback, and email delivery, call recording and compliance, emergency calls (999/112) and location accuracy, fax-to-email or ATA functionality, CRM screen-pop integrations, and failover/disaster recovery scenarios.

Go-Live Checklist

Item	Owner	Status
All DDI numbers ported and tested	VoIP provider / Cloudswitched	Required
Network QoS configured and verified	Network engineer	Required
All handsets deployed and registered	On-site engineer	Required
IVR menus tested end-to-end	Project manager	Required
Call recording functional & compliant	Compliance officer	Required (regulated industries)
Emergency services (999) tested	VoIP provider	Required
Staff training completed	Training lead	Required
Rollback plan documented & communicated	Project manager	Required
Old PBX kept powered on (parallel period)	IT manager	Recommended
Monitoring dashboards live	VoIP provider / Cloudswitched	Recommended

Step 9: Migration Readiness Scorecard

Before pulling the trigger, score your organisation against these readiness criteria. If any area falls below 70%, address it before proceeding to go-live.

Step 10: Post-Migration Support & Optimisation

The migration doesn’t end at go-live. The first 30 days are critical. Call patterns emerge that weren’t visible during testing, users discover edge cases, and the network experiences real-world load for the first time.

The First 30 Days

During the first week, your provider should be monitoring call quality metrics — Mean Opinion Score (MOS), jitter, packet loss, and registration stability — in real time. Any call quality issues need to be triaged and resolved within hours, not days. At Cloudswitched, we provide dedicated post-migration support with a named engineer for the first 30 days of every project.

During weeks two through four, focus on optimisation: review call analytics to identify bottlenecks in ring groups or queues, adjust IVR menu options based on actual caller behaviour, and fine-tune voicemail greetings and out-of-hours routing. This is also the time to decommission the old PBX — but only after you’re fully confident the new system is stable.

Ongoing Management

VoIP platforms are software-defined, which means they evolve. New features, security patches, and integrations arrive regularly. Unlike a PBX that sat in a cupboard unchanged for a decade, your VoIP system should be actively managed. Ensure you have:

• A named account manager or support contact
• Proactive monitoring of call quality and system health
• Regular reviews (quarterly is ideal) to align the phone system with business changes
• A clear escalation path for critical issues
• Documentation kept up to date as users, DDIs, and call flows change

Common Pitfalls & How to Avoid Them

Having managed hundreds of PBX-to-VoIP migrations for UK businesses, Cloudswitched has seen every pitfall there is. Here are the most common — and how to avoid each one.

Pitfall	Impact	Prevention
Skipping the network assessment	Poor call quality, jitter, dropped calls	Always perform a full network audit with VoIP-specific testing before migration
Forgetting analogue devices	Fax machines, alarms, lifts stop working on go-live day	Audit every port on the PBX; plan ATAs or replacements for each analogue device
Leaving SIP ALG enabled	One-way audio, registration failures, intermittent call drops	Disable SIP ALG on every firewall and router in the path
Inadequate staff training	Flood of support tickets, user frustration, perception that VoIP is “worse”	Train in small groups 2–3 days before cutover; provide quick-reference cards
No rollback plan	Panic if something goes wrong, no way to revert quickly	Keep the old PBX powered on for 2–4 weeks post-migration; document rollback steps
Porting paperwork errors	Number port rejected, migration date slips	Triple-check account holder name, address, and account number match the losing provider’s records exactly
Underestimating bandwidth needs	Calls degrade during peak internet usage	Implement VLAN segregation and QoS; consider a dedicated SIP trunk or SD-WAN
Ignoring emergency call requirements	999 calls routed to wrong location; potential legal liability	Register correct site addresses with your VoIP provider; test 999 before go-live

Why the PSTN Switch-Off Makes This Urgent

BT and Openreach have confirmed that the Public Switched Telephone Network (PSTN) and ISDN will be switched off across the UK by January 2027. This is not a voluntary upgrade — it is a hard deadline. After that date, traditional phone lines will simply stop working. Every ISDN2, ISDN30, and analogue line in the country will be decommissioned.

If your business still relies on ISDN trunks feeding a PBX, you are on a countdown. The closer you get to the deadline, the harder it will be to secure engineering resources, get porting slots, and receive the attention your project deserves from providers who will be overwhelmed with last-minute migrations.

Choosing the Right VoIP Provider

Not all VoIP providers are created equal. Some are pure resellers who will sell you licences but leave you to configure the system yourself. Others are full-service providers who will manage the entire migration end to end. When evaluating providers, consider:

• UK-based support — can you speak to a real engineer, not a script-reading helpdesk overseas?
• Project management — will they assign a dedicated project manager to your migration?
• Network expertise — can they assess and upgrade your LAN/WAN, or do they only handle the VoIP platform?
• Number porting experience — how many ports have they managed? Do they handle the paperwork for you?
• Post-migration support — what does support look like after go-live? Is there a hypercare period?
• Platform flexibility — can they support hybrid deployments if you need on-premise survivability alongside hosted VoIP?
• Compliance — can they meet your industry’s regulatory requirements (FCA call recording, NHS DSP Toolkit, etc.)?

At Cloudswitched, we handle every aspect of the migration — from the initial network assessment and call flow design through to handset deployment, number porting, staff training, and post-migration support. We don’t disappear after go-live; we stay with you as your ongoing communications partner.

The Business Case Beyond Cost Savings

While the 40–60% cost reduction is compelling, VoIP migration delivers strategic benefits that go far beyond the monthly phone bill:

• Remote & hybrid working — staff can use their office extension from anywhere via a softphone app on their laptop or mobile. The office number follows them.
• Scalability — adding a new user takes minutes, not an engineer visit. Opening a new office means plugging in phones and connecting to the internet — no PBX installation required.
• Unified communications — voice, video, instant messaging, presence, and screen sharing on a single platform.
• Advanced analytics — real-time dashboards showing call volumes, wait times, missed calls, and agent performance.
• Business continuity — if your office is inaccessible (fire, flood, pandemic), calls automatically route to mobiles or home workers. No single point of failure.
• Integration — VoIP platforms integrate with Microsoft Teams, CRM systems, helpdesk tools, and workflow automation platforms.

Summary: Your Migration Roadmap

Migrating from a traditional PBX to VoIP is a significant project, but it is entirely manageable with the right planning, the right partner, and a structured approach. To recap the essential steps:

1. Audit your current PBX infrastructure, hardware, and call flows thoroughly.
2. Assess your network’s readiness for VoIP traffic — bandwidth, QoS, switching, and cabling.
3. Choose your migration strategy — phased, big bang, or parallel running.
4. Initiate number porting early and triple-check the paperwork.
5. Build the VoIP platform to replicate and improve your existing call flows.
6. Train your staff in small groups, close to the cutover date.
7. Test exhaustively against a structured test plan before going live.
8. Go live with a rollback plan in place and keep the old PBX powered on.
9. Optimise during the first 30 days based on real-world call data and user feedback.
10. Decommission the old PBX only when you are fully confident in the new system.

The PSTN switch-off is coming, legacy PBX hardware is reaching end-of-life, and the business case for VoIP has never been stronger. The only question is whether you migrate on your terms — planned, tested, and stress-free — or scramble at the last minute when every provider in the country is overloaded.