Quick answer
A hosted phone system is far more resilient than the office phone system it replaces, for one structural reason: the system is not in the office. If the building loses power, internet or access, the platform in the provider's data centres keeps answering your numbers and can route calls automatically to mobiles, to another site or to a message, while staff carry on working from the app on 4G or from home. What a hosted system needs from you is a plan for the things it cannot control: a UPS on the router and switch so the office phones ride out short power cuts, a 4G or 5G failover for the broadband, correct emergency-call location information, and diverts configured before the bad day rather than during it. The one thing the old copper line did better - working with no power at all - is being retired with the network, and safety-critical devices need their own arrangements.
We build continuity into every install because the day it matters is not the day to start. This guide is the checklist. If you want your phone system's resilience reviewed, get a no-obligation quote.
What fails, and what does not
| Event | Old on-site PBX with ISDN | Hosted telephony |
|---|---|---|
| Office power cut | Everything stops unless the PBX has a UPS; analogue lines still work on exchange power | Desk phones stop unless on UPS; platform keeps answering; calls divert to mobiles; app works on 4G |
| Office internet down | PBX keeps working on ISDN | Desk phones stop unless a 4G failover takes over; platform diverts to mobiles; app works on 4G |
| Cannot get into the building (flood, fire, weather) | Phones ring in an empty office | Staff answer the same numbers from home and mobiles; reception from anywhere |
| PBX hardware failure | Everything stops until an engineer arrives | No PBX to fail |
| Provider platform incident | n/a | Rare; providers run replicated data centres; check the published uptime and status page |
| Staff illness or a site closure | Calls ring out or need manual diverts | Group membership changed in the portal in minutes; or already covered by another site |
The hosted column depends on two things you control: the diverts being configured in advance, and the office network having power and a backup connection. Both are cheap.
Configure the diverts before you need them
Every hosted platform can route calls differently when a device or site is unreachable. The rules to set up on day one:
- Main number, office unreachable: ring a hunt group of mobiles (the app or actual mobile numbers), then a voicemail box that emails the whole team.
- Individual DDIs: simultaneous ring to the user's mobile app is the default; unreachable-fallback to their mobile number as belt and braces.
- Multi-site: main number overflows to the other site's reception when the first does not answer.
- Emergency greeting: a pre-recorded "we are experiencing an issue" message that someone can switch on from the app in thirty seconds, without ringing the provider.
Test them once a year by pulling the plug on a quiet afternoon. The auto attendant guide covers hours and exception routing.
The mobile app is the continuity plan
The simplest resilience is the one already in every pocket. The provider's mobile app on staff phones uses 4G or 5G, shows the business number, sits in the same queues and needs nothing switched on when the office goes dark. For most small businesses, "everyone has the app installed and logged in" is 80 per cent of the continuity plan, and the reason hybrid teams barely notice an office outage.
Power: UPS the network
A VoIP desk phone needs power. A small UPS on the router, the switch that powers the phones (PoE) and the failover device keeps the office phones running through the short power cuts that make up most outages, typically for 30 to 60 minutes. That covers the flicker, the tripped breaker and the brief supply fault. For longer cuts, the diverts and the app take over. A UPS costs a fraction of an hour of a business without phones.
Internet: add a second path
The office broadband is the single point of failure in a hosted system. A 4G or 5G failover router takes over automatically when the main line drops, usually within seconds, and desk phones carry on; our connectivity guide to internet failover covers the options. Larger offices may run two fixed lines from different providers. Whichever you choose, make sure the failover path is tested and that voice is prioritised on it, since mobile data is more congested than fibre - see what internet speed VoIP needs.
Multi-site: failover for free
A business with two or more offices on one hosted platform has continuity built in. Calls to a site that is down overflow to a site that is up; a receptionist at either site can cover both; staff can work from whichever site has power. This used to require networked PBXs and a project; on hosted it is a routing rule.
What the provider owes you
Ask before you sign: where the platform is hosted and how it is replicated; the published uptime and the status page; how incidents are communicated; and what the provider's own continuity plan is. A reputable provider runs across multiple UK or EU data centres with automatic failover and publishes its availability. The questions are in how to choose a hosted telephony provider.
Emergency calls and safety-critical devices
Two things the plan must cover separately:
- 999. VoIP providers must support emergency calls and location information must be correct for every site and for remote workers. In a power cut, desk phones on a UPS still reach 999; staff on mobiles reach it as normal. Ofcom requires providers to offer customers who depend on the line a way to call emergency services during a power cut, typically a battery back-up or mobile fallback.
- Lifts, alarms and telecare. These must work with no power and no office network, and they are not the phone system's job. They need specialist units with their own battery and signalling path from the lift, alarm or telecare company, as covered in the PSTN switch-off explainer and the business-landlines guides on alarm systems and lift lines.
The phone system's place in the wider plan
Telephony is one line in a business continuity plan, and it is usually the easiest line once hosted. The plan should say who switches on the emergency greeting, which numbers divert where, who has the app and the portal login, where the UPS is and how long it lasts, and how staff are told to work from home. It lives with the wider business continuity and disaster recovery plan; the phone section is a page.
A continuity checklist for the phone system
- Diverts configured for office-unreachable, per site and per main number.
- Emergency greeting recorded and switchable from the app.
- Mobile app installed and logged in on every staff phone.
- UPS on router, PoE switch and failover device; runtime known.
- 4G or 5G failover installed, tested, voice prioritised.
- Multi-site overflow rules set.
- 999 location information correct for every site and remote worker.
- Lifts, alarms and telecare on battery-backed specialist units.
- Provider's status page bookmarked; uptime and incident process known.
- Portal logins held by two people.
- Tested annually.
The bottom line
Hosted telephony makes the phones the most resilient system in the building, because they are not in the building. Configure the diverts in advance, give everyone the app, put a UPS and a 4G failover on the office network, and the business stays reachable through the power cuts, broadband faults and locked doors that used to silence it. Plan separately for 999 and for the safety-critical devices that must work when nothing else does.
We build this into every install and will review an existing one: get a no-obligation quote or call 0333 006 9399. Our hosted telephony service page explains how the assessment works.
Not sure who is calling one of your numbers? Our free UK phone number checker shows which network or provider Ofcom allocated any number to, and whether the range is in service — no sign-up.
Frequently asked questions
What happens to hosted telephony in a power cut?
Desk phones stop unless the router and switch are on a UPS, but the platform in the provider's data centres keeps answering your numbers and diverts calls to mobiles or another site automatically. Staff can use the mobile app on 4G or 5G. A UPS keeps office phones running through short cuts.
Does VoIP work if the internet goes down?
Not on the affected connection, which is why a 4G or 5G failover router is worth having - it takes over within seconds and desk phones carry on. Even without one, the hosted platform diverts calls to mobiles and the app works on mobile data.
Is hosted telephony more reliable than a traditional phone system?
Yes. An on-site PBX is a single point of failure that stops working in a power cut, an internet outage (if on SIP) or a hardware fault. A hosted platform runs in replicated data centres, has no box to fail and reroutes calls automatically when a site is unreachable.
How do I keep my business phones working during an outage?
Configure automatic diverts to mobiles or another site in advance, make sure every member of staff has the mobile app installed and logged in, put a UPS on the router and switch, add a 4G or 5G failover for the broadband, and record an emergency greeting you can switch on from the app.
Can I make 999 calls on VoIP during a power cut?
From a desk phone only if the network is on a UPS; from the mobile app or a mobile phone as normal. Providers must support 999 with correct location information and must offer customers who rely on the line a way to call emergency services during a power cut.
What about lift phones and alarms when the phone lines are switched off?
They need specialist replacements with their own battery back-up and signalling path from the lift, alarm or telecare company. They should not depend on the office VoIP system, which needs power and internet. Plan them separately from the phones.
How often should I test my phone system continuity plan?
Once a year at least: pull the office connection on a quiet afternoon and check that diverts fire, the app works and the UPS holds. Test after any change to sites, numbers or providers.