Why this changed with fibre and VoIP
Old copper phone lines were powered from the exchange, so a basic corded phone kept working when the office lost power. Fibre does not carry power. The optical network terminal on the wall, the router or firewall, the network switch and every desk phone need mains power at your premises. When the power cuts out, all of them stop together.
For many offices, a short power cut is an inconvenience: computers go off too, and staff use mobiles. But for businesses that rely on taking calls, such as clinics, trades dispatch and property managers, even twenty minutes of silence on the main number can mean missed bookings or urgent calls going unanswered.
What to put on a UPS
An uninterruptible power supply is a battery unit that plugs into a wall socket and powers equipment when the mains fails. For phones and internet, the essentials are the fibre terminal (ONT), the router or firewall and the network switch that powers the desk phones. If desk phones get their power through the network cable, keeping the switch alive keeps them alive.
Leave computers, printers and screens off the phone UPS, as they draw far more power and would drain it quickly. If your server or NAS needs protection, give it its own UPS sized for it. Label the UPS and what is plugged into it, so nobody plugs a heater into it later.
Sizing and runtime
Runtime depends on battery size and how much power the connected equipment draws. A fibre terminal, a small business firewall and a modest power-over-Ethernet switch with a handful of phones draw relatively little, so a small UPS can keep them going for a good while. Larger switches with many phones or access points need a bigger unit.
Rather than guessing, check the power draw of each device, add them up and look at the manufacturer's runtime charts. Then test it: at a quiet time, switch off the wall socket feeding the UPS and time how long everything runs. Repeat the test once a year, because batteries lose capacity as they age and typically need replacing every few years.
Divert calls when the power stays off
No small UPS lasts all day. For longer outages, configure your cloud phone system to divert calls automatically when desk phones go offline. Calls can go to staff mobiles through the phone system's app, to a ring group of mobiles, or to voicemail. Teams calling users can answer on the Teams mobile app. Test the divert by unplugging the switch for a few minutes and ringing the main number from a mobile. It should reach the backup destination within the time you expect. Make sure the people receiving diverted calls know why their mobile is suddenly ringing with the office queue.
The mobile network in an outage
Mobile towers have their own battery backup, but during widespread or long outages some towers may run out of power or become congested. That is another reason to keep calls simple during an outage and to have a plan for longer events, such as moving key staff to another location with power.
If your area has frequent outages, as many rural areas do, consider a larger UPS for the network gear and a backup internet connection that does not depend on the same local infrastructure, such as Starlink with its own UPS. A generator is a bigger step; connecting one to a building's wiring is work for a registered electrician.
Write it into your continuity plan
Note what is on the UPS, how long it lasts, how calls divert and who is responsible for checking equipment comes back cleanly when power returns. Routers and switches sometimes need a restart after a long outage, and someone should check the phones register again. Keep a printed one-page procedure in the comms cupboard and a copy on staff phones. When the lights go out, people should not need to search a server that is also switched off to find out what to do. A short drill once a year keeps the plan familiar.
Do not overlook surges when the power returns. Equipment can be damaged by spikes as supply is restored, especially in areas with storm damage to lines. Most UPS units filter the supply, which protects the network gear behind them. For other equipment, quality surge-protected power boards are a cheap safeguard, and switching off sensitive devices during a long outage until supply settles avoids unnecessary risk.
In short
- Fibre and VoIP need mains power at your premises.
- Put the ONT, firewall and PoE switch on a small UPS.
- Test runtime yearly and replace batteries every few years.
- Set calls to divert to mobiles automatically when phones go offline.
- Write the outage procedure down and keep a printed copy.