PSTN vs VoIP: how to choose before the January 2027 switch-off

For most of the last century the choice of business phone system was no choice at all: you rented lines from BT and put a PBX in a cupboard. That network is now being switched off. This article compares PSTN and VoIP on the things that matter to a business buyer, explains what the January 2027 deadline means in practice, and sets out how to decide.

What PSTN and ISDN are

The public switched telephone network (PSTN) is the analogue copper network that connects premises to local exchanges. Voice travels as an electrical signal on a dedicated line. ISDN (ISDN2 and ISDN30) is the digital business service built on the same infrastructure, typically feeding an on-site PBX with multiple channels and a block of direct dial numbers.

Both are billed per line or per channel plus call charges, both need an engineer to add capacity, and both tie the phone system to the building. They were reliable because they were simple, and simple is also why they cannot do what businesses now expect: follow staff out of the building, integrate with the software they use, or tell anyone how many calls went unanswered.

What VoIP is

Voice over Internet Protocol converts speech into data packets and carries them over your internet connection to a hosted platform, which routes them to the person or number being called. The phone system is a service in the provider's data centres. Users connect from IP desk phones, softphone apps on PCs and mobiles, or Microsoft Teams. Our what is VoIP guide covers the mechanics.

PSTN vs VoIP compared

Comparing the two in detail

Cost

PSTN costs are mostly fixed and grow with every line. VoIP costs track headcount and typically bundle the features a PBX charged extra for. For a single-site firm of ten with simple needs the difference may be modest. For a business with three depots paying line rental at each and call charges between them, VoIP is usually markedly cheaper. Budget for the one-off costs too: handsets, any router or switch upgrade and porting fees. The observed ranges are in how much VoIP costs in the UK.

Reliability

This is where PSTN's reputation comes from and where the comparison has changed. A landline works when the broadband is down and the power is off. A VoIP desk phone does not, unless there is a 4G backup and a UPS. That is the whole case for PSTN, and it is answered by design: business-grade connectivity, quality of service on the router, a mobile backup at the office, and automatic divert to mobile apps. Once those are in place, a hosted platform is more resilient than a PBX in one building, because a flood, a power cut or a cut cable at the office no longer stops calls being answered. Details are in is VoIP reliable for business calls.

Features and flexibility

A PBX gives you calling. A hosted platform gives you a call-handling system: an auto-attendant, queues with reporting, call recording, voicemail-to-email, presence, mobile apps, and integration with Microsoft Teams and your CRM so callers' records appear on screen. The flexibility matters more than any single feature. Staff can be added or moved in minutes, a new office is a configuration change, and routing can be adjusted from a browser rather than by an engineer.

Security

PSTN is physically isolated but offers no monitoring, no authentication and no visibility of misuse. VoIP is exposed to the internet and therefore needs configuring: strong authentication, multi-factor login, encrypted calls, restricted international dialling and limited admin rights. Configured properly, it offers far more control and a full audit trail. Configured badly, it is a toll fraud target. This is why VoIP belongs inside your IT provider's remit rather than with a separate telecoms reseller.

What about keeping the PBX with SIP trunks?

Some businesses with a recent on-site PBX ask whether they can keep it and simply replace the ISDN with SIP trunks, which deliver the same channels over the internet. Technically, yes, and for a PBX bought in the last few years it can be a sensible interim step. The drawbacks are that the PBX remains a single point of failure in one building, it still needs maintenance and eventually replacement, remote and hybrid staff are still an afterthought, and the features stay whatever the PBX can do. Most businesses that look at the numbers over three to five years find hosted VoIP cheaper and simpler than trunking an ageing PBX, and it avoids doing the migration twice.

Power, emergency calls and vulnerable users

The one thing PSTN did that VoIP does not is draw power from the exchange, so a corded analogue phone worked in a power cut. With VoIP, desk phones stop when the switch loses power unless there is a UPS, and staff move to the mobile app. For emergency calls, VoIP providers must give 999 access and hold a registered address for each number. Ofcom expects providers to offer customers who depend on their landline a solution, such as a battery back-up unit, that keeps emergency calls available for a period during a power cut. For a business with lifts, a monitored alarm or lone workers, that needs designing in, and the devices themselves usually need a mobile-network path independent of the office broadband.

The switch-off changes the question

Until recently the question was "should we move to VoIP?". Now it is "when, and how carefully?". Openreach stopped selling new PSTN and ISDN lines from September 2023, and BT has rescheduled the withdrawal of the network to 31 January 2027. Ofcom's guidance on moving landline phones to digital technology explains what customers can expect from their providers during the migration.

For a business the consequences are:

• Every analogue line and ISDN circuit needs a replacement before that date. After it, they stop working.
• Anything else that uses a phone line, from alarm panels and lift phones to card terminals and fax, needs its own migration.
• Broadband that was delivered over a phone line moves to SoGEA (broadband without a voice line) or full fibre.
• Businesses that leave it late will be competing for engineer and porting capacity with everyone else who left it late.

The practical checklist, device by device, is in the PSTN switch-off: what UK businesses must do.

How to decide

Given the deadline, the decision for most businesses is about platform, timing and scope rather than PSTN versus VoIP. Work through:

1. What lines and devices do you have? Audit numbers, ISDN channels, and every device plugged into a phone socket.
2. When do contracts end? Line rental and PBX maintenance notice periods set your earliest sensible move date.
3. Is the network ready? Connectivity, router, switches and cabling at every site.
4. What do you need the phones to do? Queues, recording, Teams integration, CRM screen-pops, mobile working.
5. Who supports it? One provider for the network and the phones avoids the "it is the broadband, no it is the platform" argument. That is how Dig IT's VoIP and business phone systems service is structured.

A very small business with one line and no other dependencies can move in a fortnight. A multi-site firm with ISDN30, an alarm receiving centre contract and a dozen card terminals needs a project, and the sensible time to start it is now rather than in the autumn of 2026. The process itself is set out in how to switch from a landline to VoIP.

What to do next

If you still have analogue or ISDN lines, the first job is an audit of what depends on them. Dig IT starts with that audit and covers connectivity, network readiness, 8x8 deployment, porting and the migration of alarms, lifts and terminals as one plan. Talk to an engineer about your lines before the deadline sets your timetable for you.