Let's face it – you should always have a Plan B, or even a Plan C if needs be. Voice Over IP communication relies predominantly on the viability of the Internet, and at times this connection is severed through outages (due to inclement weather, scheduled maintenance, etc). The definition of Failover is any backup operational mode in which the functions of a system component (in this case, your phone system) is assumed by a secondary component. In its purest form, it's simply insurance that moves things to a standby system, ready to take up arms like a technological reserve for the sake of business continuity. Here's a rundown of a few events that would require failover:
Unplanned failure – Generally triggered by hardware failure, software failure, or a loss of connectivity.
Planned failure – Typically induced as part of Operating System or application upgrade, or server hardware maintenance process
There are an abundance of methods to ensure that calling is never interrupted. One of the more popular means, includes Failover to POTS. This method is exactly what it sounds like, with an outage in a VoIP system being rerouted to a Plain Old Telephone System/landline. It takes very little to power a phone (between 6 and 12 volts at about 30 amilliamps), with the low voltage being supplied via the copper wires underground. The fact these lines are buried, makes them impervious to the elements, with phone companies running generators at their own offices to ensure POTS lines are always functioning no matter the circumstance.
Another way to build a failover plan is right through your service provider, who may offer a fill in the blanks form in which you point out where you'd like to reroute calls and they take care of the technical side of things. This is most common to hosted providers, with companies like ShoreTel leveraging modular voice switches to backup their voice solution. In fact, most providers offer failover, with SIP-enabled devices (through Failover DNS) being forwarded to your specified failover number in the event they timeout or are unresponsive. The playing field is open with the rerouting of numbers, especially with users of open source communications platforms like Asterisk setting complex scripts (dial plan macros) to specify exactly how calls will be routed. There are simply too many mention in this piece alone, each with their own strengths and weaknesses.
With that stated, it's beneficial to have several backup providers to route calls through, whether it be a landline, your cell phone, or even a third-party that offers this service (if your primary carrier does not). Creating multiple backup plans will help your business keep ticking, especially in the midst of disaster. Failover systems have become a huge focus following the aftermath of Hurricane Sandy, and just a month prior, the FCC created outage reporting rules for providers to protect VoIP users moving forward. As the technology continues to mature, so do the devices that make it a favorable and safe method of communication for the ensuing future.