What Happens to Your Phone System in a Power Cut?

When the power goes out, your router, VoIP adapter, and any desk phones lose power along with everything else in the building, so calls stop working unless you've got a backup in place. This is a genuine difference from a traditional landline, which drew a small amount of power directly down the phone line itself and often kept working during a power cut even when everything else in the office went dark. VoIP doesn't have that built-in fallback, so it's worth understanding what actually happens and what your options are before it happens to you.

Why Doesn't VoIP Work Like a Traditional Landline During a Power Cut?

Old-style analogue phone lines carried a small electrical current from the telephone exchange itself, which is why a basic corded phone would often keep ringing during a power cut, even if nothing else in the building had power. VoIP calls are internet-based, which means they depend on your router, any VoIP-specific hardware, and your internet connection all staying powered — none of which happens automatically once the electricity goes off.

What Actually Stops Working?

Without power, everything involved in making or receiving a VoIP call goes down together:

• Your router and broadband connection
• Any VoIP adapter or session border controller
• Desk phones plugged into mains power
• Softphone apps on a desktop computer that's also lost power

Calls simply stop connecting from that point, both incoming and outgoing, including 999 calls, unless a backup solution is in place.

What Are Your Options for Staying Connected?

Battery backup (UPS) for your router and phone equipment An uninterruptible power supply keeps your router and VoIP hardware running for a limited time after a power cut, often somewhere between 30 minutes and a few hours depending on the unit. This is the most common fix for a genuine power outage, buying enough time to get through a short interruption.

A mobile broadband failover connection Some businesses set up a secondary internet connection using 4G or 5G, which kicks in automatically if the main broadband connection drops. This protects against internet outages generally, not just power cuts, and can be paired with a UPS to keep it running.

Automatic call diversion to mobile numbers Many hosted phone systems allow calls to divert automatically to a mobile number if the system detects it can't reach the office — worth setting up in advance rather than trying to configure mid-outage.

Mobile app calling on a separate connection If your phone system includes a softphone app, staff can often continue taking calls from a mobile phone using its own data connection, independent of the office's power and broadband entirely.

Our guide to staying connected to VoIP during power outages goes through these options in more practical detail.

What Does Ofcom Require Providers to Offer?

Ofcom requires VoIP providers to offer at least one hour of backup access to emergency services during a power cut, for customers identified as depending on the service. This is a regulatory minimum aimed primarily at protecting vulnerable residential customers, rather than a guarantee that your entire business phone system will keep running normally for an hour. Our guide to 999 calls on VoIP covers this requirement in full.

For a business, relying solely on the regulatory minimum isn't usually a sound continuity plan — it's designed as a safety net, not a business continuity solution.

How to Plan for a Power Cut Before It Happens

• Fit a UPS to your router and any critical VoIP hardware, sized for how long you realistically need to stay connected
• Set up automatic call diversion to mobile numbers as a fallback, tested in advance rather than assumed to work
• Check whether your phone system includes a mobile app that works independently of office power and broadband
• Make sure whoever's responsible for IT or telecoms knows the plan, so nobody's improvising during an actual outage
• Review this alongside your wider business continuity and disaster recovery planning, rather than treating phone resilience as a separate problem

Frequently Asked Questions

Will my 999 calls still work during a power cut? Not without a backup power source. Ofcom requires providers to offer at least one hour of emergency-only backup access for at-risk customers, but this isn't automatic for every business setup — it's worth checking exactly what your provider offers.

Does a UPS keep my whole phone system running, or just part of it? It depends what's plugged into it. A UPS covering just your router and VoIP adapter keeps calling itself alive, but desk phones plugged separately into the mains will still lose power unless they're on the UPS too.

Is a mobile broadband failover connection worth it for a small business? It depends on how often your area experiences outages and how critical uninterrupted calling is to your business. For businesses that can't afford any downtime, it's a relatively low-cost way to remove a single point of failure.

Can I set up call diversion to my mobile permanently, just in case? Yes, and it's often worth testing this in advance so you know it works, rather than trying to configure it for the first time during an actual outage.

A few hours spent planning for a power cut is far cheaper than the cost of missed calls during one. Request a quote if you'd like T2K to review your business continuity setup, or read our guide to what Ofcom requires for emergency calling on VoIP for the regulatory side of this.