You say that...
I wonder if it's some sort of converter box from fibre to POTS with a backup battery that is fried, as IIRC there is a similar thing that is installed near the incoming line if you go full fibre so that in the event of a power cut the phone line will still work with a plain telephone (one of those ones that gets it's power from the line and has no fancy functions other than caller id/presets) for calls.
Although if that is the case, I'd hope that it did something more to alert BT there was a fault than beep in the hope that someone will call about it before the battery/power goes.