> 5. Placing ferrite cores on the phone cables.
Do either of the phone lines in question have DSL on them?
If so, a ferrite core (which will block common-mode RF
signals) probably won’t help much, if at all. DSL is a
differential-mode signal, and its frequency content starts
down in the tens of kHz. Ferrite cores are usually intended
to block much higher frequency interference, and won’t have
enough inductance to help much with DSL signals.
What I would suggest, is that you get yourself a couple
of DSL “microfilters”… plug them into the A400P FXO
ports, and plug the lines into the filters. These sorts
of filters are designed specifically to block DSL differential-
mode signals from getting into analog-phone circuits, and
they will also be fairly effective against other forms
of low-frequency-RF noise.