What happens to VoIP in a power cut or internet outage?

Does VoIP work in a power cut?

The honest answer is that the equipment in your office does not. VoIP desk phones and your broadband router both run on mains electricity, so when the power goes off, the handsets go quiet and the local internet connection drops with them. There is no getting around the physics of that. This is the one genuine trade-off compared with an old copper landline, which could carry a basic call using power fed from the telephone exchange.

But that is only half the story, and it is the half that misleads people. A VoIP phone system is not your handset. Your number and your call routing live in the provider's cloud, not in the box on your desk. So while the phone on the desk cannot ring, the system that decides where calls go is still running, miles away, on its own resilient power and network. That is what you build your continuity plan around.

What keeps your business reachable when the line is down?

Because your number sits in the cloud, you can tell the system what to do the moment the office becomes unreachable. This is called failover, and it is the single most useful resilience feature VoIP gives you. Inbound calls reroute automatically, usually within seconds, so a customer ringing during an outage simply gets answered somewhere else without knowing anything is wrong.

Failover option	What it does	Best for
Auto-divert to mobiles	Inbound calls forward to staff mobiles when the office line is unreachable	Every business, no extra hardware
Mobile and web app	Staff keep taking calls from a phone or laptop on 4G, 5G or home broadband	Remote and hybrid teams
Backup 4G or 5G router	Keeps the internet, and the desk phones, running if broadband drops	Offices that need full phones during outages
UPS battery backup	Powers your router and handsets through short power cuts	Brief or frequent outages
Multi-site routing	Calls auto-route to another office or team	Businesses with more than one location

Most small businesses do not need any hardware to be safe. Setting calls to divert to mobiles costs nothing extra and covers the common case of a broadband blip or a short power cut. Bigger or more critical operations layer on a backup connection and a UPS so the phones themselves keep working. Either way, you decide the plan in advance and the system runs it automatically. For more on what a stable connection looks like, see our guide to internet speed for VoIP.

Can I still call 999 in a power cut on VoIP?

VoIP supports 999 and 112 emergency calls in normal use, and the system registers your location. The catch is the same as for any other call: if your power or internet is down, the office handset will not work, so you cannot dial 999 from it. The simple, reliable answer is to always keep a charged mobile phone to hand, which is good practice in any office regardless of phone technology.

Providers in the UK have specific duties here. They must offer at least one solution that lets customers contact emergency services for a period during a power cut, and they must take extra care of people who rely on their landline for safety, such as users of personal alarms. For a typical business, a mobile phone covers this, but it is worth confirming the arrangements for any vulnerable staff or for connected safety devices.

Is VoIP really less reliable than a landline?

For a single call during a power cut, the old copper landline had an edge, because it drew power from the exchange. For everything else, VoIP is more resilient, not less. A landline tied your number to one building, so a cut line or a fire meant your number was simply unreachable until an engineer fixed it. With VoIP, the same event just triggers a divert, and your calls carry on from mobiles or another site.

The bigger point is that the comparison is ending. The PSTN and ISDN switch-off on 31 January 2027 means the analogue network that powered those copper lines is being retired across the UK. Every business is moving to internet-based calling, so the practical question is not landline versus VoIP, but how to set up VoIP resilience well. Done properly, with failover and a backup connection, a cloud system keeps you reachable through events that would have taken an old landline off the air completely. You can see the full comparison in our guide to VoIP versus landline.