If your business still runs on a traditional phone line — the kind that plugs into a socket on the wall and works even when the power’s off — that line has a retirement date. Openreach is switching off the old analogue and ISDN network on 31 January 2027, and every business still on it needs to have moved before then. That deadline is what’s pushing most of the VoIP-versus-landline decisions happening right now, so it’s worth understanding what you’re actually choosing between, not just which one a phone salesperson tells you is cheaper.
Why this isn’t really optional
The UK’s traditional phone network — PSTN for the ordinary copper line, ISDN for the older multi-line business version — runs on infrastructure BT and Openreach have been trying to retire for years. The switch-off date has moved more than once, but Openreach and industry figures now describe 31 January 2027 as fixed, with no further delay expected. Around 1.9 million business and residential lines in the UK were still to be migrated as of early-to-mid 2026, a number that’s falling steadily as providers work through it.
The detail that matters most for businesses specifically: ISDN2 and ISDN30 lines — the multi-channel lines many offices and call-handling businesses rely on — have no migration path at all. They will simply stop working. There’s also no consumer-style safeguard for business lines during the transition, and the wholesale cost of keeping a legacy line running has been rising sharply ahead of the deadline as an incentive to move early. In practice, this means the question for most SMEs isn’t “should we switch to VoIP” so much as “when, and to what.”
What you’re actually comparing
A traditional line — analogue or ISDN — carries your call as an electrical signal down a dedicated copper circuit from your premises to the exchange, and from there through the phone network. The line itself is basic infrastructure: it doesn’t know or care about the internet, and it kept working during a power cut because the phone company fed it a small current down the same wire.
VoIP — Voice over Internet Protocol — does away with that dedicated circuit. Your call is converted to data and sent over your broadband connection, the same way a video call or a file download travels. The “phone system” becomes software: a handset, an app, or a desk phone that’s really just a small computer talking to your provider over the internet. Because of that, a VoIP number isn’t tied to a physical wire at all — which is where most of the practical differences, both good and bad, come from.
Where the savings actually come from
The cost gap between the two is real, though the figures you’ll see quoted by VoIP resellers tend to sit at the generous end of the range — treat any specific percentage as illustrative rather than a number you can bank on for your own business. The pattern behind the figures holds up, though: a traditional line combines ongoing line rental with per-minute call charges and, for a multi-line office system, a physical PBX box that needs buying, housing and maintaining. VoIP replaces most of that with a single per-user monthly fee that usually bundles calls (often including UK landline and mobile minutes) and needs no on-site hardware beyond handsets or headsets, because the “exchange” is run in your provider’s data centre rather than a cupboard in your office.
| Item | Traditional line | Hosted VoIP |
|---|---|---|
| Line rental, per user/month | £25–£45 | £5–£15 |
| On-site hardware (multi-line system) | £3,000–£8,000 upfront | £50–£100 per handset |
| Call charges | Metered, per minute | Usually bundled |
| Engineer call-outs / maintenance | Charged separately | Typically included |
The saving tends to be largest for businesses that make a lot of calls (because per-minute charges disappear into the bundle) or that run several lines or extensions (because there’s no PBX box to buy and no engineer to call when it needs fixing). A sole trader on a single line making occasional calls will see a smaller gap in absolute terms — but for that same sole trader, VoIP often still wins simply because there’s nothing to install, and the “system” can move with a phone number that no longer depends on a specific building.
What changes about where your number “lives”
This is the part that catches people out, because a traditional phone number and a VoIP number behave differently in ways that aren’t obvious until they matter.
A landline number has always been tied to a physical exchange — that’s why UK area codes (01/02 numbers) are geographic: the code tells you which exchange, and therefore roughly where, the line is. Move your office and, traditionally, you’d lose that number or have to pay to keep it forwarded. A VoIP number breaks that link. It’s just an entry in your provider’s software, so you can keep your original area code number even after moving premises, run a number with a different area code to look locally based somewhere you don’t have an office, or use a non-geographic 03 number that behaves the same wherever you are. For a growing business, that flexibility is genuinely useful — but it also means a phone number is no longer a reliable clue to where a business is actually based, worth bearing in mind if you use area codes yourself to judge who’s calling.
The more important side of this is emergency calls. Ofcom requires phone providers, VoIP included, to make accurate caller location available automatically to the 999 and 112 emergency services wherever that’s technically possible, at no charge. For a wired landline, that location is inherent — the exchange knows where the wire ends. For VoIP, the provider has to be told your address, because the number itself no longer carries that information. In practice, this means giving your VoIP provider your correct site address when you set the service up, and updating it if you move — something a landline never required you to think about, because the wire did it for you. It’s a small admin step, but it’s the one genuinely safety-relevant difference between the two, and worth checking rather than assuming is handled.
The other practical consequence of moving voice onto the internet: a VoIP phone depends on your power and your broadband connection. A basic analogue line kept working in a power cut because the phone network supplied its own current; a VoIP handset and your router both need mains power, and the call needs a working internet connection, to do anything at all. Most providers offer a mobile app or call-forwarding failover for this, but it needs setting up in advance — it isn’t automatic the way the old line’s resilience was. Worth thinking through if your phone line doubles as a safety line — a site intercom, an alarm line, a lift phone, or simply the only way a lone worker can reach the office.
You don’t have to get a new number
Switching provider — landline to VoIP, or one VoIP provider to another — doesn’t mean giving up the number you’ve already got. Porting a number to a new provider is a right for end-users under Ofcom’s rules, and it’s routine practice: your new provider submits the request and deals with the old one on your behalf, so it’s not something you have to negotiate yourself.
What they’ll ask for is usually simple: your current provider’s name, your account number (a recent bill will have it), and a signed letter of authority confirming you’re the account holder and want the number moved. Timescales vary more than most guides let on — a single geographic number typically ports in around a week to ten working days, while a multi-line system, a block of DDI extensions, or a non-geographic 03 number can take two to four weeks. Porting is officially a “best efforts” process rather than a guaranteed one, so it’s worth starting it two to three weeks ahead of your target switch date rather than the week before.
A few things commonly slow a port down or stall it: the address or account details you give the new provider not matching exactly what the old provider has on file, an alarm or monitoring service (a “Redcare” line, for instance) still attached to the number, or the line being registered as a different type than it actually is. Worth cross-checking a recent bill against what you’re about to submit before starting. On the day itself there’s typically little to no downtime, but the old line is normally ceased once the port completes — so it’s a one-way move rather than something you can trial in parallel.
How this plays out in practice
Sole trader: a one-person plastering business
Runs entirely from a mobile in practice, but keeps a landline number on the website and van signage because it reads as more established than a personal mobile number. The line is barely used for calls — it’s really just a number that needs to exist and ring through somewhere.
Here VoIP is close to a straightforward swap: a low-cost hosted number that forwards straight to the mobile costs less than the old line rental, needs no engineer visit, and — because the number isn’t tied to an exchange — can move house or change van livery without the number ever changing. The one task not to skip: giving the VoIP provider a correct home or business address for 999 routing, since there’s no longer a wire doing that automatically.
Limited company: an 8-desk accountancy practice
Runs an ISDN-based office phone system with a receptionist extension, four staff extensions, and a fax line kept mainly for one legacy client. The ISDN30 circuit has no migration path and will stop working outright at switch-off, so this business doesn’t have the option of waiting it out.
The move to hosted VoIP replaces the on-site PBX box entirely, drops the extensions onto individual logins staff can use from a desk phone, a laptop or a mobile with no difference in behaviour, and removes the per-minute call charges the practice was paying on client calls. The main jobs beyond the technical switch: confirming the office address is registered correctly with the new provider for emergency-call location, and finding a fax-to-email replacement for that one legacy client rather than assuming a physical fax line will still exist to move.
What’s worth checking
- Whether your current line is analogue, ISDN2 or ISDN30 — the ISDN lines have no migration path and need a plan sooner, not just before the January 2027 deadline.
- If you want to keep your existing number, gathering your account number, a recent bill and a signed letter of authority, and starting the port two to three weeks ahead of your target switch date rather than the week before.
- Whether your broadband connection is reliable and fast enough to carry voice calls cleanly alongside whatever else it’s already doing — VoIP call quality is only as good as the connection underneath it.
- What happens to calls during a power cut or broadband outage, and whether the provider’s mobile app or call-forwarding failover is actually set up, not just available.
- Whether the address held by your new provider for 999 location purposes is correct, and who’s responsible for updating it if you move premises.
- Whether any other equipment shares your current line — a card machine, an alarm system, a fax, a lift phone — since these often need their own separate migration plan and aren’t automatically covered by a voice-only VoIP switch.
The Gaffer’s take
The businesses that get caught out here aren’t usually the ones who switch too early or too late — they’re the ones who treat it as a pure IT swap and forget the number itself needs looking after: port it in good time if you want to keep it, and make sure whoever answers a 999 call from your new number actually knows where you are.
Figures and the switch-off date in this guide reflect publicly available information as of August 2026. The industry timeline has shifted before, so it’s worth confirming the current deadline directly with your own provider if you’re planning a move close to it.