After Google searching for a bit I believe it's an [Optical Network Terminal](https://forums.att.com/t5/U-verse-TV-Installation/What-is-the-meaning-of-quot-ONT-quot/td-p/5241165). If you Google "AT&T Optical Network Terminal" it pulls up some pictures of this same box as well as what I assume are newer versions of it.

It's a GPON ONT, Gigabit passive optical network, optical network terminal converts infrared light on the fiberoptic line to your home back into Ethernet basically

The alarm part is for the battery. Incase it dies or is discoed.
( the battery is for the phone system when there is a power outage.
I install a similar system by Calix in Minnesota)

After Google searching for a bit I believe it's an [Optical Network Terminal](https://forums.att.com/t5/U-verse-TV-Installation/What-is-the-meaning-of-quot-ONT-quot/td-p/5241165). If you Google "AT&amp;T Optical Network Terminal" it pulls up some pictures of this same box as well as what I assume are newer versions of it.

Moved into apartment, found this on wall.