What happens to business phone lines after the PSTN switch-off?

If you run a business with a few phone lines, an office phone system, or a main number that customers have called for years, the PSTN switch-off is worth understanding properly rather than worrying about. It's a planned, nationwide change to how phone lines work, and the business version has more moving parts than the home one: multiple lines, a phone system, direct dials, and often a handful of connected services that quietly depend on an analogue line. This guide explains, in plain terms, what changes for a business, what replaces the old lines, what happens to your numbers and connected kit, and how to plan the move calmly. If you want the home version instead, our residential PSTN switch-off guide covers that.

The PSTN switch-off means old business phone lines have to move to digital services. For most businesses, that means replacing analogue or ISDN-style line setups with hosted voice, SIP, or another IP-based phone system, while making sure broadband, numbering, handsets, alarms, and continuity are all planned together.

What does the PSTN switch-off mean for a business phone system?

PSTN stands for the Public Switched Telephone Network: the analogue phone system that has carried UK calls for well over a century. It's the copper-line technology behind a traditional phone plugged into the wall, and behind many of the line types that feed business phone systems too. That network is old, expensive to maintain, and being retired across the whole country, with a national deadline of 31st January 2027.

For a business, the key point is that the lines themselves are what's changing, not your phone numbers and not your ability to make and take calls. Whether you have a small group of analogue lines, an ISDN setup, or analogue lines feeding an on-site phone system, those line types are being switched off and the service moves onto an IP-based equivalent that runs over a data connection. Your calls stop travelling down dedicated copper and start travelling over your broadband or a dedicated circuit instead.

The practical experience of making a call doesn't change much: someone picks up a handset, dials, and talks. What changes underneath is the technology carrying the call, the kit on your desks, and (often) the broadband that supports it. The work is to identify everything that currently relies on an old line, then move it in one planned piece rather than discovering a dependency after the lines have gone.

What usually replaces old business phone lines?

Old analogue and ISDN-style business lines move to an IP-based replacement. There are a few common shapes this takes, and which one fits depends on your size, how your team works, and whether you want to keep any existing hardware.

• Hosted voice. The phone system itself lives in the cloud rather than as a box in your office, and handsets or apps connect to it over the internet. This is the natural fit for most businesses, because there's no on-site system to maintain and users can work from anywhere. You can read a plain definition on our hosted PBX glossary entry.
• SIP trunks. If you have an on-site phone system you want to keep, SIP trunks deliver the calls to it over your data connection in place of the old ISDN or analogue lines. The system stays, the lines feeding it change.
• Microsoft Teams Phone. For businesses already living in Microsoft 365, calling can be added into Teams so the phone system and the collaboration tools are one and the same.

All of these are forms of VoIP (Voice over Internet Protocol): voice carried over a data connection rather than a dedicated phone line. The choice between them is a genuine decision rather than a formality, and we come back to how to think about it further down. The deeper detail of each option lives on our business phone systems page; here the aim is simply to understand the shape of the replacements.

What happens to numbers, handsets, alarms, and other connected services?

This is the part that catches businesses out, because a phone line in an office often does more than carry voice calls. Several things hang off it, and each needs accounting for before the old lines cease.

Numbers. Your main number, your direct dials, and any geographic numbers can almost always be ported to the new service, so customers reach you on the same numbers as before. The job is to make sure every number that matters is on the list, including older direct dials that are easy to forget, so nothing is stranded when the old lines go.

Handsets. Some existing desk phones can carry over, others can't, depending on whether they support IP calling. Part of planning the move is working out which handsets stay, which are replaced, and whether some users are better served by a softphone app on a laptop or mobile instead of a desk phone at all.

Connected services on analogue lines. This is the one not to skip. Intruder and fire alarms, door entry systems, lift emergency lines, card payment terminals, and fax machines have all traditionally used analogue lines, and they do not automatically keep working once those lines cease. Each one needs to be identified, then migrated to a digital alternative or moved to a different solution. Lift lines and alarm signalling in particular are safety-related, so they deserve early attention rather than being discovered at the end. Walk the premises, note everything plugged into a phone socket, and flag it before the migration is scheduled.

Do I need hosted VoIP, SIP, or a full phone system?

There isn't a single right answer, because the best fit depends on how your business actually runs. The useful way to choose is to start from a few honest questions rather than from the technology.

• How many people make and take calls, and where do they sit? A small team across one office has different needs from a business with remote staff, multiple sites, or a contact-handling function. Hosted voice tends to suit distributed and growing teams because it isn't tied to a box in one building.
• Do you have an on-site phone system worth keeping? If you've recently invested in a capable on-site system, SIP trunks let you keep it and just change the lines feeding it. If the system is old, replacing it with hosted voice is usually cleaner than propping it up.
• Do you live in Microsoft 365 already? If your team runs on Teams all day, putting the phone system inside Teams can mean one tool instead of two.

The honest position is that most businesses moving off old lines land on hosted voice, because it removes on-site hardware and bends easily as the business changes, but SIP and Teams Phone are the right answer for plenty of others. Rather than pick a product in the abstract, it's worth talking through your setup with someone who can map it to the options. Our business phone systems page sets out what each route involves in detail.

What should a business check before migrating?

A smooth migration comes down to knowing what you have before you change it. The work isn't difficult, it's just a matter of being thorough, and a short checklist covers most of it.

• List every line and number. Main number, direct dials, any spare or forgotten lines, and what each one is actually used for.
• Identify the connected services. Alarms, door entry, lift lines, card terminals, and fax. Note anything plugged into a phone socket and decide its digital path early.
• Check the broadband underneath. If your connection rides on an old analogue line, it ceases when the numbers port, so it has to move to SOGEA or full fibre at the same time. You can read short definitions on our SOGEA glossary entry and FTTP glossary entry.
• Confirm there's enough connection for the calls. IP-based voice needs stable upload headroom for the number of simultaneous calls you make, so the connection has to be sized for it, not just present.
• Decide handsets and users. Which desk phones stay, which are replaced, and who is better off on an app.

Because the phones and the broadband so often move together, it helps to plan them as one piece of work. Our business broadband sits alongside the phone move for exactly this reason, and our full fibre vs FTTC guide explains the connection options if you're weighing them up. If your premises also doubles as a home office, our home office broadband covers that case.

When should a business start planning the move?

The national deadline is 31st January 2027, but the sensible answer for a business is to start the planning well before then rather than aiming for the wire. There's no need to rush the change itself, but giving yourself room means the move happens on your timing, around your busy periods, and with connected services and numbering checked properly rather than under pressure.

Starting early also matters because some premises are caught by a local exchange change on top of the national switch-off, which can bring the relevant dates forward or set a particular migration path. That's a separate situation, and our help page on an exchange closing and forced migration covers it in detail. If you suspect your premises may be affected, flag it early so the timing is planned around the right dates rather than the national deadline alone.

The reassuring part is that none of this has to be project-managed alone. A good provider will help you list what you have, map it to the right replacement, port the numbers, sort the broadband, and account for the connected kit, then schedule the move when it suits the business. The aim is a change that feels planned and calm, not a scramble before a deadline.

Frequently asked questions about the business PSTN switch-off