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So I have FIOS phone, and my phone has a loud background hum noise when I'm on a call. When I plug a phone directly into the ONT, there is no hum. So clearly, there is something going on on my end.
But 2 issues:
1) It wasn't always there - I think this hum only started up a month or so ago, and
2) when I UNPLUG the ONT (so it's running on emergency battery power), then there is no hum on any of the lines.
Hum is easy to cause. Probably a wire disconnected partially, grounded, or something else in your enviroment that changed is being picked up. If possible trace the wires looking for problems.
Possibly something going wrong with the BBU transformer, since you mention it goes away when you are on battery power.