How to migrate from an on-premise PBX to the cloud

Why move from an on-premise PBX to the cloud?

An on-premise PBX is the traditional setup: a physical phone system installed in your building that you own, power and maintain. It served UK businesses well for decades. The reason so many are now moving to the cloud is not that the old system suddenly stopped working, but that the ground underneath it has shifted.

The biggest single driver is the deadline. The UK's old PSTN and ISDN networks are being switched off on 31 January 2027. Any phone system that depends on those traditional lines has to move to an internet-based service before then. Buying new on-premise kit that still relies on PSTN or ISDN is a dead end, because the lines it needs are being retired.

Alongside the deadline, three practical pressures push businesses towards cloud. Ageing hardware reaches end of support, so spares and security patches dry up and maintenance contracts climb. Remote and hybrid working does not fit a box bolted to one building, where a number is tied to a desk. And the running cost of owning, insuring and servicing on-site hardware adds up against a simple monthly per-user fee. Cloud removes the hardware, the maintenance and the line rental in one move.

On-premise PBX vs cloud phone system

The two approaches do the same job, taking and routing calls, but the ownership model is completely different. This table sets them side by side on the points that matter to a UK business making the decision.

Factor	On-premise PBX	Cloud phone system
Upfront cost	High. Hardware, licences and installation, often £1,000+ per user upfront.	Low or none. No hardware to buy; setup is usually remote.
Ongoing cost	Maintenance contract, line rental, spares, eventual replacement.	Per-user monthly fee, typically £7 to £30 across the UK market.
Maintenance	Your responsibility, or a paid support contract.	Handled by the provider; updates are automatic.
Scaling	Add cards, licences or hardware; can mean an engineer visit.	Add or remove users instantly online.
Remote and mobile working	Limited; numbers tied to physical desks.	Built in; the same number rings on desk, mobile app and browser.
Future-proofing	Depends on PSTN or ISDN lines being retired by 2027.	Already runs over the internet; ready for the switch-off.
Advanced features	Often extra hardware or licences for recording, queues, IVR.	Recording, AI transcription, IVR and AI agents included or added in software.

For most small and medium UK businesses, cloud wins on every line except the rare cases where heavy regulation or a very specific integration ties an organisation to on-site hardware. Even then, the 2027 deadline forces the underlying lines onto IP.

What does cloud replace your PBX with?

Moving to the cloud is not a like-for-like swap of one box for another. Instead of a physical PBX, your phone system becomes a service that runs in the provider's data centre. You reach it through whatever device suits each person: an IP desk phone, a mobile app on iOS or Android, or a softphone in the web browser. The same number can ring on all three at once.

Everything the old PBX did, and a good deal it could not, moves into software. Auto-attendant menus, ring groups, call queues, business-hours routing, voicemail-to-email, hold and transfer are all configured online rather than programmed into hardware. Because the call is now data, the system can also record and transcribe calls, summarise them with AI, and log calls and notes to your CRM. Some platforms, Voxora included, add an AI Receptionist that answers calls 24/7, handles common questions and routes the caller to the right person or to a human. None of that needs an engineer or a new box on the wall.

How do you migrate from a PBX to the cloud without downtime?

A well-run migration keeps your phones working throughout. The key is to build and test the new system before you touch the old one, then switch over in a single planned step. Here is the sequence most UK businesses follow.

1. Assess what you have. List your lines, numbers, users, desk phones and any special call routing. Note any connected devices on the same lines, such as alarms, lifts, door entry, card machines or fax, so nothing is forgotten.
2. Check your broadband. Each call uses about 100 kbps in each direction, so a modern FTTP or SoGEA line easily handles a busy office. Confirm you have headroom for the number of simultaneous calls you expect.
3. Choose a provider and plan. Match users to the right plan, confirm UK calls are inclusive, and check the features you need are covered. Get a tailored quote rather than guessing.
4. Set up users and call routing. Build your menus, ring groups, queues and business-hours rules in the new system while the old PBX is still live.
5. Order or port your numbers. Start the port of your existing numbers. The new provider manages it and your old line stays active until the port completes, so there is no gap.
6. Run in parallel and test. Use the new system alongside the old one for a week or two. Make and take test calls, check routing and confirm staff are comfortable with the apps and phones.
7. Go live and retire the old lines. Once the numbers have ported and everything is tested, switch over fully and cancel the legacy lines and maintenance contract.

Run this way, the changeover is a non-event for callers. Your numbers ring as they always did, just on a system that is cheaper to run and ready for the future.

Keeping your numbers and reusing handsets

Two questions come up on every migration: what happens to my numbers, and what happens to my phones. Numbers are the easy part. Through porting you keep your existing geographic, 03 and 0800 numbers when you change provider. You do not cancel the old line first; the new provider ports the number across and the old line stays live until it does. Porting is usually free and typically takes a few working days to around two weeks depending on the number type and the losing provider.

Handsets vary. Many IP desk phones can be re-provisioned to register with a new cloud system over SIP, which can save replacing them, though this depends on the make, model and firmware. Older analogue or digital phones tied directly to the legacy PBX generally cannot be reused and are replaced with IP phones, the mobile app or a browser softphone. The practical answer is to ask the new provider to check your exact handset models before you migrate, so you know upfront whether to keep or replace them.