I've been meaning to write up a blog article on this topic, because there is a lot of confusion.
For our traditional Fusion Broadband+Phone service, the broadband is xDSL (ADSL2+ or VDSL2), and the voice service is line-powered POTS voice. It's backed up by the batteries and large generator systems in the central office where our equipment is located. Assuming your phone itself does not require power (cordless ones do, as do some with answering systems), a Fusion voice service of this type should just always work, regardless of premise power, or internet access. And if you have a battery for the modem/WiFi router, your Internet would remain online too. We have many customers who were "camping" at home, watching Netflix by candlelight, using a small battery UPS to keep their router up. But that's go no bearing on the phone service, to be clear.
By contrast, some AT&T POTS lines are served out of remote neighborhood huts or remote terminals, and these generally have batteries but not generators. This is likely why some AT&T "landlines" lost service during the recent long public safety power shutoff events. And the same goes for their UVerse service, where both phone and internet come from a small "VRAD" cabinet, the newer tan colored cabinets with cooling systems on them. The batteries eventually die, resulting in an outage of internet and phone, even if the customer has invested in a UPS on premise. This also affected Sonic members who use our resold AT&T service, IP Broadband, which uses the uVerse network.
Comcast ended up in the same boat, because their cable nodes require power, and are backed up by batteries. These are often on poles - you'll see a roughly 2.5' square box often labeled "Alpha", the maker of the power system, scattered around town. After the 2017 fires, I saw that Comcast added generators either at the base of the poles, or in at least one case, built a small platform up there for a generator. Then these must be fueled daily, etc - it's a challenge.
Back to us and our customers, Sonic's fiber service also benefits from a lack of dependency on power at remote locations. Because our fiber network is a "passive optical network", the cabinets you see on poles or on the ground which are Sonic splitter locations do not have any electronics in them: they're simply passive light splitters, taking the light from one fiber and splitting it to go to around 30 homes. But with fiber, the voice service is delivered using voice over IP, and premise power is required for voice functionality. So, for our members with Sonic fiber, during a power outage they would need to use a UPS or generator to provide power to the optical network terminal (small white Sonic labeled box that outputs Ethernet and voice). And to keep their WiFi up, power for the router that connects to it.
During the 2017 and 2018 fires and the 2019 fires and PSPS events, Sonic's traditional Fusion & Fiber to the home networks were unaffected, and Sonic customers phone service was not interrupted by these events. But for internet access and for fiber-based VoIP service, a premise UPS or generator would be required.