Cloud phone systems and on-premise PBX look similar in a demo and behave differently over five years. The short version.
On-premise means a physical PBX in your building, one-off capital cost, and full control. Cloud means the PBX lives in a provider’s data centre and you access it over the internet, one monthly bill per user. Which fits your business depends on how you work now, what you already own, and how close you are to the 2027 PSTN switch-off deadline.
This guide is for UK SMEs weighing up cloud vs on-premise phone systems in 2026. It covers the quick side-by-side, the real-world cost picture over five years, the PSTN switch-off timeline, and a five-question test that gets most businesses to the right answer in one meeting.
The short answer: how they compare
Both do the same job. Route inbound and outbound calls, handle voicemail, connect handsets to trunks. What changes is where the hardware sits, how you pay, and what you have to look after.
| Factor | On-premise PBX | Cloud phone system |
| Where it lives | Physical hardware at your site | Provider’s data centre |
| Upfront cost | £5,000 to £30,000 CapEx | Zero or minimal |
| Monthly cost | £3 to £8 per user (SIP trunks + maintenance) | £10 to £30 per user (all-in) |
| Setup time | 3 to 8 weeks (survey, install, config) | 3 to 10 days |
| Remote work fit | Needs VPN or Session Border Controller | Native, works from any internet connection |
| Maintenance owner | Your business or your MSP | The provider |
| Scaling up | New licences plus hardware capacity check | Add users in the admin console |
| PSTN switch-off in Jan 2027 | Needs SIP trunks or full replacement | Compliant by design |
| Best fit | Existing hardware not due replacement, specific compliance needs | New setups, hybrid teams, growing businesses |
What on-premise PBX gives you
An on-premise PBX is a rack-mounted server in a cupboard at your office. Once bought, it is yours. You control the dial plan, the call recording, where the data sits, and how it integrates with your CRM. Depreciation runs over five to seven years, and total cost is heavily front-loaded.
The fit is strongest where you already own PBX hardware that has three or more years of useful life left, where regulatory or industry requirements mandate on-site call storage, or where the internal team has the capability to manage the platform, in addition, this can be a reasonable route for businesses with unusually complex integrations that a mainstream cloud vendor cannot match out of the box.
What a cloud phone system changes
A cloud phone system, sometimes called hosted PBX or cloud telephony, lives in the provider’s infrastructure. You subscribe per user per month. Your team uses a mix of desk phones, softphones on laptops, and mobile apps, all against the same extension and the same voicemail.
The provider handles patching, feature releases, and resilience. VoIP is the underlying technology that carries voice over IP networks. Cloud telephony is one deployment model that uses VoIP, hosted by a provider.
This model has become the default for UK SMEs since 2020 for one clear reason. Hybrid work fits it.
A softphone on a laptop is the same phone at home, at the office, and at a customer site. Numbers port with the person. Provisioning a new starter takes ten minutes.
The PSTN switch-off changes the maths in 2026
Openreach is retiring the analogue PSTN and ISDN networks by 31 January 2027. Any on-premise PBX still connected to those legacy trunks needs SIP trunks or full replacement before the deadline. As of early 2026, Openreach reported roughly 500,000 UK business lines still on legacy phone services, which is why price rises and stop-sells on WLR products have accelerated through the year.
In June 2026, Openreach clarified that January 2027 is not a hard cut-off. Migrations continue exchange by exchange, with the bulk of the volume moving in a phased rollout.
What that means in practice for UK businesses. If you are still using ISDN or PSTN today, you are already in the migration window. As a result, businesses now have two main options. Firstly, you can upgrade your existing PBX with SIP trunks. Alternatively, you can use this opportunity to move to a cloud-based system.
Cost: five-year total, not month-one
Comparing month one is misleading. Compare over five years for a fair picture.
On-premise, 20 users, five years: £8,000 to £20,000 upfront hardware and install, £3 to £8 per user per month for SIP trunks and licences, plus £1,000 to £2,500 a year on maintenance and support. Total: around £28,000 to £65,000, depending on how the hardware refreshes.
Cloud, 20 users, five years: £15 to £25 per user per month, zero to minimal upfront. Total: around £18,000 to £30,000. No refresh, no maintenance contracts, feature releases included.
Cloud tends to be cheaper on total cost for most SMEs and more predictable on cash flow. In some cases, on-premise can be the cheaper option. For example, this may apply if you plan to extend the life of existing hardware for another two to three years. However, you should also have a clear plan in place for the SIP migration.
Reliability and who owns the failure modes
Both platforms fail sometimes. What changes is who is responsible.
On an on-premise PBX, you own the power supply, the UPS, the on-site internet, the hardware itself and the SIP trunk to the outside world. When something breaks, your team or MSP needs to fix it. If you have the right capability in place, the response can be fast. However, without that capability, resolving the issue may take much longer.
On a cloud phone system, the provider owns the platform. What is left on your side is the internet. If your internet connection drops, your calls will also go down. However, a mobile failover on the softphone app can help keep your team connected.
A well-designed internet setup with a backup path removes most of that risk. Our SD-WAN and backup internet setup piece covers the resilience pattern for this.
Remote work and the softphone question
This is where cloud pulls ahead for most modern businesses. In addition, a cloud phone system gives every user access to a laptop softphone and mobile app. If needed, they can also use a desk phone, while all devices remain connected to the same extension.The team can take a call from a client site, from home, or from an airport, and the caller cannot tell the difference.
However, achieving the same setup with an on-premise PBX requires VPNs, Session Border Controllers and mobile clients. As a result, the additional operational costs can often outweigh the initial CapEx saving over three years.
How to choose: a five-question test
Most businesses can get to the right answer with five questions.
- Are you replacing existing hardware, or starting from scratch?
- Are your trunks still on ISDN or PSTN today?
- What proportion of your team works from home or on the road on a typical week?
- Do you have specific compliance or data-residency requirements for call recording?
- What is your preference: predictable monthly OpEx, or lower total cost with more variability?
Starting from scratch, hybrid team, no strong compliance driver. Cloud, almost every time.
If you already have hardware with three or more years of life left and a capable internal or MSP team, on-premise can still make sense. However, you should pair it with a clear SIP trunk migration plan and make sure the 2027 deadline is already accounted for.
Common mistakes when comparing options
- Sticking with on-premise “because we’ve always had it” without pricing the 2027 upgrade. ISDN and PSTN are going away. The upgrade is not optional, only its timing is.
- Buying a lowest-tier cloud plan then discovering feature gaps. Compare like for like: call queues, IVR, call recording, integrations, mobile app.
- Ignoring the connectivity requirement for cloud. Cloud phone quality depends on your internet. Check both bandwidth and symmetric upload before choosing a tier. Our leased line vs business broadband guide covers the underlying connection choice.
- Not planning number porting properly. Porting can take two to six weeks. Start it early to avoid the double-billing overlap.
- Missing the SIP trunk sizing question. A rough rule of thumb: one concurrent-call channel for every three to four users on a typical office workload.
Get a phone system decision session
If you would like a second pair of eyes on your current phone setup before you sign the next contract, get in touch and we will walk through what you already own, what your team’s real workload looks like, and whether the sensible next step in 2026 is a cloud move, a SIP trunk upgrade, or a phased hybrid.