said by david82 :
I don't know. The house wiring is all CAT6 and as part of trying to fix this noise issue I actually did replace all of the terminations, so pretty sure everything is properly wired.
That would certainly seem to rule out wiring then. Typically, external noise pickup or crosstalk is caused by wiring, and a noisy power supply would be more likely to effect the phone that it powered than a phone on another line.
I have seen cases where a flaky multi-line phone would cause crosstalk and noise sensitivity similar to the symptoms of wiring problems.
However, since your new power supply has fixed the noise problem, hopefully, that will be the end of it.--
History does not long entrust the care of freedom to the weak or the timid.
-- Dwight D. Eisenhower