So I've learned something.
As I was tellin my wife what I was considering doing (getting a dmarc (MPOE) DSL splitter and rewireing (re-connecting) my internal phone connections – I ended up explaining how the self-install "line filters" worked.
Then I had a revelation! In my kitchen we have a wall mounted phone that I installed the "wall-mount" filter on. This filter also had a phone socket that I then plugged my cordless phone into.
Do you get it?
That socket was a "pass-through" – no filtering was being done!
So I added a line filter between it and the cordless phone.
And now it's all quite on the phone lines.
In my own defense I gotta knock AT&T/SBC for having such lightweight instructions. They did not explain, or even show the wall-mount filter in their instruction manual.
Thanks again for all your help, and I enjoyed the tangents this thread went on.