If you need your cable co to supply the battery for you that's up to you, but you can most definitely easily do-it-yourself.
Don't know if it's just for business or what, but Cablevision supplied us with a purpose-built (proprietary cable) APC UPS when we switched to Optimum Voice a few months ago. Don't know the specs but the installer said it should last 15 hours in an outage... while I doubt that number, I'm sure it'll be good for several hours.
