You may also have phone in a power outage. The stand-alone splitters/filters don't require separate power to work; I see no reason why the splitter/filter inside the modem would be set up to require it.
I used to have a setup identical to yours. I can't remember if the phone worked when the modem was powered down or not, but it'd be pretty easy to check for yourself: just disconnect the power plug from the back of the
modem and pick up your phone.
Edited by Jon, : No reason given.
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