Does Your Business Phone Still Work in a Blackout?
An NBN-connected phone line typically stops working within minutes of a power outage unless a battery backup is installed — unlike the old copper landline, which kept running on the network’s own power. For a business that relies on its phone to take calls or bookings, that’s worth checking before the next outage, not during it.
It’s an easy detail to miss, because the switch from copper to NBN happened gradually and quietly for most businesses. The phone still rings the same way day to day, right up until the power drops and it doesn’t.
Why does my business phone go silent in a blackout?
The old copper telephone network carried its own power down the line from the exchange, which is why a basic corded phone would often still work even when the building’s electricity was out. NBN-based phone services — whether a standard NBN phone service or VoIP running over the internet connection — rely on equipment in your premises (the NBN connection box, modem/router, and VoIP adapter or handset) that all need mains power to run. No power to that equipment means no dial tone, no matter how good the underlying connection is.
NBN Co sets this out directly for residential and business customers: what happens to your landline in a power blackout.
What actually keeps a phone line working during an outage?
A few options, usually used together rather than as a single fix:
- A battery backup unit for the NBN connection box. Some NBN connection types support an optional battery backup that keeps the box (and anything plugged into it) running for a limited time after the power drops.
- An uninterruptible power supply (UPS) on your router and VoIP equipment. A UPS is a more general backup battery that can cover the rest of the equipment chain — router, modem, VoIP adapter — not just the NBN box itself.
- Mobile call forwarding as a fallback. Even with backup power in place, having calls automatically divert to a mobile number if the landline doesn’t answer means a blackout never fully cuts a business off from its customers.
How long does backup power actually last?
This varies by equipment and how heavily it’s used, and is usually measured in hours rather than days. It’s not something to assume — ask your provider or installer exactly what backup is in place (if any), how long it’s rated to last, and whether it needs to be tested or replaced periodically to stay reliable. A battery backup that was installed years ago and never checked since is a common way this fails silently.
What should a business actually do about this?
- Check what backup power, if any, is currently installed on your NBN connection box and phone/VoIP equipment.
- Ask how long that backup is rated to last, and when it was last tested.
- Set up mobile call forwarding as a fallback so a blackout doesn’t mean missed calls even if backup power runs out.
- Put a reminder in place to check the battery periodically — backup power that was fine when installed can quietly fail years later.
Frequently asked questions
Does every NBN connection type support a battery backup? Not all of them — availability depends on the NBN technology at your address. Check directly with NBN Co or your provider what applies to your specific connection.
Will a UPS keep my whole office running in a blackout? A standard UPS for phone/network equipment is sized to keep small devices like a router or VoIP adapter running for a limited time — it’s not the same as a whole-building backup generator, and isn’t meant to be.
Does a mobile phone have the same problem? No — mobile networks run on their own independent power and backup arrangements at the tower, separate from your premises’ mains power, which is part of why mobile forwarding is a useful fallback.
Who should I ask about setting this up? Your phone or internet provider can tell you what backup options apply to your specific NBN connection and equipment.
The takeaway
A business phone going silent in a blackout isn’t a fault — it’s just how NBN-based phone services work without backup power in place. The businesses that stay reachable through an outage are the ones who checked this before it mattered, not after a blackout already cost them calls.