How do you open the FiOS box-on-the-wall to take out/replace the battery?
Is this explained -- is there a user-manual -- somewhere?
https://www.verizon.com/support/consumer/battery-backup?CMP=DMC-CVZ_ZZ_ZZ_Z_DO_N_X00361
This link above should help. If you don’t have phone service the BBU is not needed.
on new installs there is a Power Reserve Unit (d cell batteries in a black case) that can be switched on and off. It might be on. Just switch it off.
