What about redundancy?
Quality business grade VoIP providers usually cover this point very well. Redundancy can be in the form of Automatic diversion to an auto fail over to another separate data service. Usually an Internet service can be used as a temporary solution.
3 Levels of failure
We’d recommend using both forms of redundancy plus adding a 3rd level so you can be certain of remaining connected to the outside world. This is where you’ll see a graphic difference between the would be’s and the service providers who’ve walked their talk for many years.