Battery Backup Unit
From FiOSWiki
A Battery Backup Unit is included with FiOS phone service, and is usually installed near the ONT. In case of a power outage or accidental unplugging of the ONT, the BBU provides up to eight hours of non-IP based telephone service. It is not designed to provide power for TV or Internet service, only emergency phone service.
The BBU contains a series of indicator lights that tell you whether your service is being powered by your home's electricity or the battery.
