“What happens when the internet goes down?” is the fair question every VoIP buyer asks. The honest answer is that nothing happens automatically — and that configuring what should happen takes about twenty minutes and almost nobody does it.
Failover happens above your building, not inside it
This is the part that gets misunderstood. If your connection drops, nothing you own can help — whatever you have on site is unreachable by definition. The diversion has to be held by whoever controls the number, upstream of the fault.
That is good news: it means failover works even when the office has no power at all, and it costs nothing but configuration.
Layer one: a destination on every number
Each inbound number should carry a second destination that takes over when the primary route is unreachable. A mobile, a second site, or voicemail — in that order of preference.
Set it per number rather than once for the account. The sales line and the accounts line rarely want the same fallback, and a single catch-all usually ends up ringing one unfortunate person's mobile for everything.
Layer two: a second path out
For outbound, the equivalent is a second route. A 4G or 5G backup connection on the router costs little and carries voice comfortably — voice needs about 100 kbps per call, which is nothing on mobile data.
Test that it actually takes over. A backup connection nobody has failed over to is a theory, not a plan.
Layer three: soft clients on phones
If the office is unreachable but staff have working mobile data, a softphone app on their phone reaches the platform directly. Extensions keep working, internal transfers keep working, and outbound calls still present the company number.
This is the cheapest layer by a distance because the hardware is already in everyone's pocket. It needs to be installed and logged in before the outage, though — nobody is downloading and configuring an app during one.
Layer four: put the PBX where the outage is not
An on-premises PBX has a single point of failure: the building it is in. Power cut, flood, connection fault — anything that reaches the building reaches the phone system.
A cloud PBX moves that box somewhere with redundant power and multiple connections. Your office losing its line then stops being a phone system outage and becomes a phone system that your staff are temporarily reaching from their mobiles instead.
Write down the answer before you need itMost failover plans fail because the diversion existed but nobody knew it was there. One page: what diverts where, who can change it, and how. Kept somewhere reachable when the office network is not.
Test it on purpose
Unplug the WAN cable on a quiet afternoon and ring your main number from a mobile. You will learn more in five minutes than from any amount of documentation.
Three things to check: how long before calls divert, where they actually land, and whether the person answering knows why they are getting them. That last one catches more problems than the first two.
Do the first layer today
Layers two to four are projects. Layer one is a form field. If your numbers have no failover destination set, that is twenty minutes of work that turns a total outage into a mild inconvenience — and it is the single highest-value thing on this page.
Failover destinations are part of every VOIPNOX number, not an upsell.
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