This sort of thing has become much more of a problem with modern electronic telephones. You'll often find that if you plug in an old 500 or 2500-set that you won't have any trouble.
You have to stop the RF signal getting into the phone in the first place. Once it's in and been rectified into audio, nothing will stop it. As the offending signal is coming from one particular station, the best approach might be to install a notch filter.