Battery backup for digital landlines

Last reviewed: 2026-03-04

Why you might need battery backup

If your landline goes through the broadband router, it will not work in a power cut unless the router has power. A battery backup keeps the router (and any fibre box) running for a limited time, so you can still make and receive calls.

Battery backup is most important if:

• You depend on the landline and mobile signal is poor or absent
• You use telecare, a personal alarm, or medical equipment that connects through the router (use the device risk checker to find out which devices are affected)
• You are elderly, disabled, or in another situation where losing phone access is a safety risk

Types of battery backup

Provider-supplied backup

Some phone providers offer a small battery backup unit designed to power the router during an outage. Under Ofcom's landline migration rules, providers must offer at least one solution for emergency calling during power cuts. This might be a battery unit, a mobile handset, or call diversion.

Ask your provider: "What battery backup or power-cut solution do you offer?" Some provide it free; others charge or only offer it to Priority Services Register customers.

Mini UPS (uninterruptible power supply)

A mini UPS is a small battery unit that sits between the wall socket and the router. When the power goes off, it switches to battery automatically. No setup is needed beyond plugging it in.

A typical mini UPS for a broadband router provides 2 to 8 hours of backup, depending on the battery size and what is connected. If you also need to power a fibre box (ONT), the runtime will be shorter.

USB power bank with adapter cable

Some routers can be powered from a USB power bank using an adapter cable (sometimes called a USB to barrel-jack cable). This is a lower-cost option, but not all routers support it. Check your router's power requirements before buying.

Larger UPS

A full-size UPS (the kind used for desktop computers) will also work, but is larger, heavier, and more expensive than needed for just a router. Only consider this if you also want to keep other equipment running.

What to connect

To keep your landline working, power these devices from the battery backup:

1. Broadband router or hub: always needed
2. Fibre box (ONT): if you have full-fibre (FTTP), this is the small box where the fibre cable enters your home. It needs power too.
3. Phone adaptor: if your provider uses a separate adaptor between the router and phone

You do not need to connect the phone itself to the battery backup if it is a corded phone (it draws power from the router's phone port). Cordless phone base stations do need mains power, but many have a short internal battery.

How long will it last?

Runtime depends on the battery capacity and how much power the connected devices draw. Rough guide:

• Provider-supplied backup: typically 1 hour (Ofcom minimum expectation)
• Small mini UPS (20-30Wh): roughly 2-4 hours for a router alone
• Larger mini UPS (50-100Wh): roughly 4-8 hours for a router alone
• Adding a fibre box (ONT): reduces these times by roughly a third

These are estimates. The actual runtime depends on the specific equipment.

Setting it up

1. Plug the battery backup into the wall socket. Let it charge fully before relying on it (usually a few hours).
2. Plug the router (and fibre box, if applicable) into the battery backup instead of directly into the wall.
3. Test it. Switch off the power at the wall. Check that the router stays on and you can make a phone call. Then switch the power back on.
4. Test again every few months. Batteries degrade over time. Set a reminder to check it still works.

Common questions

Will my internet work too?

If the router has power and your broadband connection is still live (the exchange or cabinet has its own backup), then yes, internet and landline should both work. In a very widespread outage, the exchange equipment could also lose power, in which case the internet will be down regardless of your home battery.

Do I need battery backup if I have good mobile signal?

For most people with reliable mobile signal, a charged mobile phone is sufficient for emergencies. Battery backup for the router is mainly for people who depend on the landline, for example because of telecare, poor mobile signal, or business needs.

What about a generator?

A portable generator can power a router, but generators have safety risks (carbon monoxide, fuel storage) and are much more expensive and complex than a battery unit. For keeping a phone line working, a battery backup is simpler, safer, and usually sufficient.