Here you go, up to 8 hours backup, 5 hours talk time. Wouldn't work well for the wilds of Sonoma County, where our power has gone missing for up to 7 days. We're about 50 miles North of San Francisco.
XFINITY Voice relies on the power in your home. A backup battery for your leased voice modem could help maintain your service in the event of a power outage, allowing you to stay connected for up to 8 hours with 5 hours of talk time.
According to posters in
Arris TG862G Backup Battery - Comcast Help and Support Forums, you have to buy that battery for $40 after you install the box.
Pat