There's not a lot of useful information in your post.
My best wild guess is the battery in your ONT is ready to be replaced.
If you want a more helpful answer, provide more details about the "white box" including the status of any lights on it or related equipment.
Its probably the ONT telling you it thinks you need a new battery. That may not be the case. Search on line for resetting the FIOS battery. I did it a week ago and it stopped the beeping (I also tested the battery and it was fine). I beleive it involves unplugging ONT box, then unplugging the battery within the box, waiting a couple minutes, and replugging battery, then ONT.