- A traditional PSTN phone draws power from the exchange over the copper line, so it usually keeps working in a mains power cut; a VoIP phone depends on the router and home power and stops when the electricity does.
- Ofcom requires providers to ensure that vulnerable customers who rely on their landline have access to a means of calling emergency services during a power cut, free of charge.
- Ofcom's guidance describes a solution that should last at least one hour for an uninterrupted call to the emergency services as a minimum benchmark for resilience.
- Openreach's all-IP migration is scheduled to complete in 2027, after which the legacy PSTN network is withdrawn and analogue phone service over it ends.
- A battery backup unit only powers the phone path; full-fibre lines where the cabinet or street equipment loses power may still cut off unless the provider's network elements are themselves backed up.
VoIP needs mains power to work, so it fails in a power cut unless a battery backup keeps the equipment running. Ofcom obliges providers to offer a free backup solution to customers who depend on their landline.
Last reviewed: June 2026
Why a VoIP line goes dead when the power does
For more than a century the analogue telephone had a quiet superpower: it was powered down the line itself. The copper pair running from the local exchange carried a small voltage that lit up the handset, rang the bell and carried the call. Because that current came from the exchange, where standby batteries and generators keep the equipment alive, a corded analogue phone would generally keep working even when the street, the house and the whole neighbourhood lost mains electricity. Many people only discovered this during a storm, when the lights were out but the phone still gave a dial tone.
Voice over Internet Protocol, or VoIP, works on a completely different principle. The call is converted into data packets and carried over the broadband connection. That means the voice path now depends on a chain of mains-powered devices: the broadband router, any separate adapter or analogue telephone adapter that the handset plugs into, and the network equipment in the street and at the exchange. The moment the household loses power, the router shuts down and the dial tone vanishes. A cordless DECT handset is doubly exposed because its base station also needs mains power. This is the single most important practical change households face as the old network is retired.
What the PSTN switch-off changes
The Public Switched Telephone Network, the legacy analogue system, is being retired as Openreach migrates customers to all-IP services. Openreach's published programme schedules the all-IP migration to complete in 2027, after which voice services that used to run over the copper PSTN are delivered digitally instead. In practice this means the landline number that used to plug into a wall socket now runs through the broadband router, and the resilience that came free with the old exchange-powered line no longer applies automatically.
This is not a fault or a downgrade chosen by any one provider; it is an industry-wide change driven by the ageing of the copper network. The consequence for power resilience is universal. Every household moving from analogue to digital voice loses the exchange-powered backup unless a deliberate replacement is put in place. That is exactly why Ofcom set protections around the change, and why providers are required to identify customers who could be left without a way to call for help.
The Ofcom obligation and who must provide backup
Ofcom has set out guidance on protecting access to emergency organisations when there is a power cut at the customer's premises. The core expectation is that providers must ensure all customers have access to the ability to contact emergency services during a power cut. For most people, a mobile phone provides that fallback. But Ofcom recognises that some customers cannot rely on a mobile: they may live in an area with poor mobile coverage, they may not own a mobile, or they may depend on the landline because of age, disability or a telecare alarm.
For those at-risk customers, the responsibility sits with the communications provider that supplies the voice service. The provider must offer, free of charge, a solution that enables the customer to contact the emergency services for at least the duration Ofcom describes as a reasonable minimum, which its guidance frames as an uninterrupted call lasting at least one hour. The provider is also expected to identify which of its customers are at risk and to make the solution available proactively rather than waiting to be asked. Households who think they qualify should contact their provider and explain why they depend on the landline, so the provider can record the need and arrange the appropriate equipment.
VoIP power resilience options and what they cost
There is no single fix that suits every home. The right choice depends on how long resilience is needed, whether a mobile is a realistic fallback, and whether the household has a telecare device or other dependent equipment. The table below sets out the common options and the typical mechanism behind each. Figures are described as ranges because actual costs vary by provider and model, and the provider-supplied battery backup unit is free of charge where the customer qualifies under Ofcom's protections.
| Option | How it works | Typical backup duration | Cost |
|---|---|---|---|
| Provider battery backup unit | Battery keeps router or adapter running so a 999 call can be made | At least one hour benchmark | Free for eligible at-risk customers |
| Mobile phone fallback | Call 999 from a charged mobile when the landline is down | Limited by battery and signal | No extra hardware cost |
| Uninterruptible power supply (UPS) | Standby battery powers router and adapter together | Depends on capacity, often longer than one hour | Bought separately, varies by model |
| Hybrid or fallback service | Device routes 999 over a mobile network when broadband fails | Depends on mobile coverage | Varies by provider package |
One limitation is important to understand: a battery backup unit protects the equipment inside the home, but it cannot guarantee that every part of the wider network stays powered. On full-fibre connections the resilience question also involves the provider's street and exchange equipment. This is why providers design their solutions around the ability to make a single emergency call rather than promising continuous service through a long outage. Resilience is about reaching help, not about keeping every feature of the line running indefinitely.
What households should do before and after the switch
The practical steps are straightforward. First, work out whether anyone in the household relies on the landline in an emergency: an older relative, someone with a disability, anyone with a personal alarm or telecare pendant, or a home in a known mobile not-spot. If so, contact the provider, explain the need and ask to be recorded as a customer who requires resilience support. Providers are expected to keep a register of at-risk customers and to supply a free backup solution to those who qualify.
Second, keep a charged mobile phone available as a fallback wherever there is usable coverage, and note that a mobile is the simplest backup for most households. Third, anyone with connected equipment beyond the phone itself, such as a telecare alarm, a monitored home alarm or a lift line, should check that those services are migrated and tested, because they share the same dependency on mains power and broadband. Keeping a written note of the provider's helpline and the steps to take in an outage means the household is not improvising in the dark when something fails.
Frequently Asked Questions
Will my phone work in a power cut after PSTN switch-off?
Not on its own. A digital voice or VoIP line depends on the router and home power, so it stops working when the mains fails. To keep the ability to call for help during an outage you need a battery backup unit, an uninterruptible power supply or a charged mobile phone as a fallback.
What is a battery backup unit for VoIP?
It is a battery that keeps the broadband router or telephone adapter running for a period after the mains power is lost, so an emergency call can still be made. Providers supply these to eligible at-risk customers free of charge under Ofcom's guidance, and they are designed around supporting at least one uninterrupted call to the emergency services.
Who is responsible for providing battery backup?
The communications provider that supplies the voice service is responsible. Ofcom expects providers to identify customers who depend on their landline and could be at risk in a power cut, and to offer them a free solution that lets them contact the emergency services. Customers who think they qualify should contact their provider directly.
How long does a VoIP battery backup last?
Ofcom's guidance frames a reasonable minimum as supporting an uninterrupted call to the emergency services lasting at least one hour. Actual duration varies with the equipment, how much the line is used and the age of the battery. A larger uninterruptible power supply bought separately can extend that further, but no battery powers the wider network indefinitely.
What can I do if I rely on my phone in emergencies?
Tell your provider that you depend on the landline so you can be recorded as an at-risk customer and supplied with a free backup solution. Keep a charged mobile to hand where there is signal, and make sure any telecare alarm or monitored equipment has been migrated and tested. Note your provider's emergency helpline in advance.
