if it's the battery back up in your garage or basement, then it's probably time to replace the battery. that normally is an alarm that the battery needs to be replaced. in your TOS when you activate your fios it describes that verizon will take care of the battery for 1 year, and after 1 year you are responsible to replace it.
battery mart on the internet is the cheapest place I have found to replace it. 16 bux plus shipping.
I don't know if it's an F1 connector or an F2 connector, so on the website they give you a little guide, and you can look at your battery and see which one it has.
if you installed within a year, then call verizon and they should send you a new one at no cost.
there is also an alarm silence button, but I am not sure how long it makes it a silent alarm for. maybe a couple hours, maybe a couple days, someone else on here might be able to answer that.
you can also pick them up locally at home improvement stores. radio shack had them the cheapest when I checked last at 35 bux but you don't have to pay for shipping and handling and you get it right away. so whichever works best for you.