Here is something few providers will tell you upfront: if the power goes off, your modern phones and broadband go off with it. The old copper phone line could keep a basic handset working in a blackout because it was powered from the exchange. Today’s fibre and VoIP cannot, because the kit in your building needs mains electricity. It is not a flaw, it is just how the technology works, but it catches a lot of businesses out. Here is what actually happens, and how to stay reachable when the lights go off.
Why modern phones and internet stop in a power cut
A VoIP phone system and a fibre broadband connection both rely on powered equipment on your premises: the router, the fibre box (ONT), the network switches and the handsets themselves. Cut the mains and all of that goes dark, so the line goes with it. The traditional copper network was different: it delivered a trickle of power down the line from the exchange, so a simple corded phone kept working even when your building had no power. As the copper network is switched off and everything moves to fibre and VoIP, that old safety net is disappearing.
What this means for your business
In a power cut, without any backup in place, you lose your phones, your broadband, and anything that depends on them: card payments, cloud systems, and the ability to call out. For most offices that is a nuisance. For some settings it is a serious problem, and it deserves proper thought for:
- Lifts and emergency lines. A lift’s emergency phone has to connect even in a power cut, which is exactly why these need dedicated resilience, as we cover in our guide to the switch-off and alarms, lifts and devices.
- Alarms and door entry. Security and access systems that used to lean on the copper line need a new plan.
- Care and safety-critical sites. Anywhere a lost line is a safety issue, not just an inconvenience.
How to stay connected in a power cut
- A UPS (battery backup). An uninterruptible power supply keeps your router, fibre box and key phones running for a while after the power goes, enough to ride out a short outage or make urgent calls.
- Mobile failover. A 4G or 5G backup can keep you online when the fixed line or its power is gone, and mobiles keep working as long as the local mast has power, which it usually does.
- Divert to mobile. A cloud phone system can automatically send calls to staff mobiles the moment the office line drops, so customers still get through.
- A plan for critical kit. Lifts, alarms and the like should have their own resilient, battery-backed connection designed in, not bolted on.
The safe assumption is simple: if the power goes, your phones and internet go too, unless you have built in backup. The good news is that backup is straightforward to add once you have decided what absolutely must keep working.
The PSTN switch-off makes this urgent
This is not a far-off concern. The old copper phone network is being switched off, and with it goes the last of the “works in a power cut” assumption. Anything that quietly relied on that, a lift line, an alarm, a back-office phone, needs a deliberate plan now rather than a nasty discovery later. Our guide to the big PSTN switch-off explains the wider change.
Frequently asked questions
Do VoIP phones work in a power cut?
Not on their own. VoIP phones need a powered router and handset, so they stop when the mains goes. They keep working through a power cut only if you add battery backup (a UPS) or divert calls to mobiles automatically.
Does fibre broadband work without power?
No. Fibre needs a powered box (the ONT) and router in your building, so it goes down in a power cut unless those are on a UPS or you have a mobile failover connection.
Didn’t the old landline work in a power cut?
Yes, a basic corded phone on the old copper line was powered from the exchange, so it often worked when your building had no power. That is going away as the copper network is switched off and everything moves to fibre and VoIP.
How do I keep my phones working in a power cut?
Use a UPS to keep the router and key phones alive, add a 4G or 5G failover connection, and set your phone system to divert to mobiles if the office line drops. For lifts and alarms, build in a dedicated resilient connection.
How Synergy Max helps
We plan resilience around what your business actually needs to keep running. That means battery backup for your core kit, 4G and 5G failover so you stay online, automatic divert to mobiles, and dedicated resilient connections for lifts, alarms and other critical devices. We will tell you honestly what will and will not survive a power cut today, and fix the gaps that matter, which ties into our guide to whether your business needs a backup line.