TL;DR. PSTN is the old analogue copper phone network. VoIP routes calls over the internet. From 31 January 2027 the UK PSTN switches off — stop-sell is already live in every Openreach exchange. VoIP is cheaper (£6–£15 per user vs £18–£30 per line), offers better features, and is now the default. The migration is straightforward for voice; the care point is non-voice devices (lift alarms, PDQs, fire panels) that still dial out on copper.

What PSTN and VoIP actually are

PSTN — the Public Switched Telephone Network — is the legacy copper-and-analogue system BT and Openreach have run since the 1980s. Each line is a dedicated circuit from the premises to the local exchange. Reliable, power-independent in the handset, but expensive and unable to support modern features.

VoIP — Voice over IP — turns voice into data packets and sends them over your internet connection to a cloud-hosted phone system. The most common UK SME platforms in 2026 are Microsoft Teams Phone, 3CX, Gamma Horizon, 8x8, RingCentral and Zoom Phone.

The 2027 switch-off isn't optional

Openreach is retiring the PSTN on 31 January 2027. Stop-sell means you cannot order new PSTN lines or make most changes to existing ones — upgrades, moves, additional lines all have to go IP-based. Around 500,000 UK business lines were still on legacy copper as of February 2026. Openreach's own coverage tracks the programme. Beyond January 2027 the dial tone goes away. See our dedicated PSTN switch-off 2027 guide for the full checklist.

VoIP vs PSTN — side by side

  • Cost — VoIP: £6–£15/user/month inc. unlimited UK calls. PSTN: £18–£30/line/month + per-minute calls.
  • Scalability — VoIP: add a user in 5 minutes. PSTN: add a line in 10–30 working days.
  • Features — VoIP: voicemail-to-email, call recording, auto-attendant, mobile apps, Teams integration, CRM pop. PSTN: dial tone.
  • Quality — equivalent on a decent internet connection. HD-voice codecs (G.722, Opus) on VoIP often sound better than PSTN.
  • Reliability — VoIP depends on internet and power. PSTN sips power down the line but the exchange is going away anyway.
  • Remote working — VoIP: softphone on laptop/mobile, same number anywhere. PSTN: tied to the physical wall socket.

When PSTN is still hanging around

Voice is the easy bit. The risks are the non-voice devices still plugged into copper:

  • Lift alarm auto-dialler — must move to 4G or SIP before the line ceases
  • Fire alarm panel dial-out — same
  • PDQ / card terminals on legacy dial-up — replace with IP or mobile variants
  • Intruder alarm dialler — confirm with the alarm provider that the panel is IP-capable
  • Door entry panel with a PSTN handset connection — IP door entry swap
  • Franking machines and older fax devices

Audit every socket before the migration date. We've seen businesses switch the voice lines, only to find the fire alarm monitoring dropped silently three weeks later.

What you need for VoIP to work well

  • Internet — 100kbps per simultaneous call, low jitter, low latency. A decent FTTP or leased line with QoS is fine. Old ADSL will struggle at scale.
  • Network — managed switches with VLAN separation for voice traffic; PoE for desk phones.
  • Failover — 4G/5G backup so inbound calls route to mobiles if the primary internet fails.
  • Firewall / SBC — SIP ALG usually needs to be OFF on consumer-grade routers or it'll break calls.
  • Headsets — cheap USB headsets are where most call-quality complaints originate.

Choosing a VoIP platform

For Microsoft 365 shops, Teams Phone is usually the right answer — calls inside the same app staff already use for chat and meetings. For contact centres, retail or anywhere that needs wallboards and heavy IVR, 3CX or Gamma Horizon are stronger. We'll cover the Teams Phone vs 3CX trade-off in a separate article on 28 July 2026.

Migration — the safe sequence

  1. Audit all lines and devices (voice + non-voice)
  2. Confirm internet adequacy and add failover
  3. Pick the VoIP platform and pilot with 2–3 users
  4. Port numbers in batches (10–20 working days)
  5. Deploy handsets and softphones, train users
  6. Migrate alarms, lifts, PDQs — last, with the alarm monitoring company
  7. Cease PSTN lines only after everything else has been cut over

We handle the whole sequence as part of our business telephony service, including number porting, hardware, training and non-voice device migration. Ask for a line audit — we'll tell you exactly what's plugged into your copper and what it'll cost to replace.

FAQ

Can I keep my existing phone number on VoIP?

Yes. Number porting from BT, Virgin or any UK carrier to a VoIP provider takes 10–20 working days and is a regulated process overseen by Ofcom. Your number stays with you.

What happens to alarms, lifts and door entry on PSTN?

These are the riskiest line-off cases. Lift alarms, fire panels, PDQ card terminals and dial-home HVAC often use PSTN. Each needs replacing with a 4G, SIP or IP equivalent before the line is ceased.

How much does VoIP cost compared to PSTN?

Business VoIP typically runs £6–£15 per user per month for unlimited UK calls, against £18–£30 per line for PSTN plus call charges. For most SMEs, VoIP halves the monthly spend.

What if our internet goes down?

Inbound calls route to mobiles via forwarding rules. A 5G failover CPE keeps everything working in the first place. We configure both as standard.

Syntek Solutions Team

We migrate West Sussex SMEs from PSTN to Teams Phone, 3CX or Gamma Horizon — including number porting, handsets and the awkward non-voice devices. Ask for a line audit.