I skimmed through this so if the point is being repeated, I apologize.
The FiOS BBU battery has ALWAYS been the customers responsibility after one year, but the gray area is this: if someone has a regular , regulated phoneline (not digital voice) on FiOS, Verizon could be responsible for the battery. In NY, Ive heard that Verizon is regulated to provide at least 8 hours of battery backup for the phone service, whether its on copper or FiOS.
As an example, Verizon is migrating copper customers to FiOS, when they do this its NOT digital voice. (voice service only) Taxes and fee's are still the same, calling features, etc. Why would they be exempt from supplying 8 hours of Battery backup?
Now keep in mind, its not economical to keep a regulated phoneline over FioS, you will save a lot more on taxes and fee's by swithcing it to "digital voice" . These batteries can last as long as 6 years, thats like .25 cents a month for a new one. Set up a direct deposit savings account for your next battery.