The 2027 PSTN
switch-off, in plain English.

If you've been ignoring the noise about ISDN going away, you're not alone — most small businesses haven't engaged with it yet. The good news: the migration itself is straightforward. The bad news: every month you wait, the network slots get harder to book.

This is a written version of the conversation we have with most new clients about telephony. Skim the headlines, read the bits that apply to you, ignore the rest.

What's actually happening

BT (along with the other UK telcos) is switching off the Public Switched Telephone Network — the copper-based phone network that's carried voice calls in the UK for over a century. ISDN, the digital evolution of that network used by most business phone systems for the last 30 years, is going with it.

The official cut-off date is January 31, 2027. After that point, traditional phone lines simply don't work. The replacement is voice-over-IP — calls carried over the internet, using a protocol called SIP — which is what almost all "new" phone systems have used since around 2015 anyway.

How to tell if it affects you

If your business runs phones over the internet — softphones in browsers, Microsoft Teams calling, a "hosted" or "cloud" phone system — you're probably already fine. The migration is a problem for businesses still using:

• ISDN30 or ISDN2 trunks — typically connected to an in your officeises PBX in a comms cabinet somewhere
• Analogue lines — including ones used for fax machines, lift phones, alarm dialers and card terminals (yes, especially these)
• Old "WLR" lines — Wholesale Line Rental analogue lines, usually for single phones in shops and small offices

If you're not sure, take a look at the bottom of last month's phone bill. Anything that says ISDN, WLR, PSTN, BTN or "trunk lines" is in scope. If it just says VoIP or SIP, you're probably already there.

The four things you actually need to do

1. Audit every line, including the boring ones

The mainline phones are obvious. What gets people in trouble is the forgotten copper line in the back of the server cupboard that turns out to be carrying the alarm signal, or the lift emergency line, or the spare fax in finance that one customer still uses.

Every analogue line needs identifying. Each one falls into one of three buckets:

1. Replace with SIP (most phones, fax-to-email)
2. Replace with a mobile alternative (lift phones, lone-worker safety devices, some alarms)
3. Replace with a special-purpose ATA (Analogue Telephone Adapter — for the few legacy devices that genuinely need a dial-tone)

2. Pick a SIP network and a phone system

You don't have to pick the same vendor for both. The network provides the trunks and the numbers; the phone system is what your team actually uses. Common patterns:

• Hosted PBX — both the trunks and the phone system live in the network's cloud. Easiest to deploy, easiest to support. The right answer for most small businesses.
• Microsoft Teams direct routing — your phone system is Microsoft Teams; the SIP network provides the trunks. About the same price as Microsoft's native calling plans, but more flexible too. The right answer if you're already a Teams shop.
• Internet phone lines into your existing office phone system — keeps the phone system you've already paid for, replaces only the lines. Tactical move if the PBX has years of life left.

3. Plan the number port carefully

Porting a number is a paperwork-heavy process, especially with some of the obstinate networks. The mechanics are routine but there are sharp edges:

• Most ports complete in 2–4 weeks. Multi-line and TUPE ports take longer.
• Always run in parallel — keep the old line working until the new one is fully tested.
• Check the bill name matches reality. Numbers ported from "Jane Smith t/a Smith Solicitors" when the business is now "Smith Solicitors Ltd" cause weeks of delay.
• The network you're porting from must release the number; they have a regulatory obligation, but they'll make it as awkward as the rules allow.

The cost of waiting isn't the price of the migration. It's the lead time on network slots, which gets longer every month from here to 2027.

4. Decide what happens to existing handsets

Modern Yealink, Polycom and Cisco deskphones can usually be re-provisioned for the new network — they're standard SIP devices already. Anything more than ~8 years old, or anything that's vendor-locked to an old PBX (some Avaya, some Mitel), gets replaced.

Don't skip the audit. Replacement handsets are £80–£250 each; budgeting for "we might have to replace 30 of them" is sensible.

What this all costs

Honestly: less than people fear. The recurring monthly cost almost always drops compared to the equivalent ISDN setup. The one-time project cost depends on the size of the setup:

• A 10–25 person single-site business: typically £1,500–£4,000 project, with monthly costs down by 20–40%
• A 50–100 person business: £5,000–£15,000 project, similar percentage drop in monthly
• Multi-site, complex IVR, contact-centre features: scoped per case, but the savings scale

None of this is reason to leave it. The reason to start now is that network installation lead times stretch as the deadline approaches. We're already seeing 8–12 week porting timelines for some destinations; that will get worse.

What we'd do if it were us

1. This month: audit your bills. Find every PSTN, ISDN, WLR and analogue line. Get them written down.
2. Within 90 days: pick a target architecture (hosted PBX, Teams direct routing, or SIP-to-existing). Get quotes.
3. This year: cut over. Run in parallel for at least 30 days before disconnecting the old lines.
4. 2026: clean up. Use the lift in capability — call recording, analytics, CRM integration — that you couldn't do on ISDN.

If you want the conversation rather than the writeup, we're happy to do a 20-minute audit call. It's genuinely free — we'd rather you came to us with a clean question than a panicked one.