The lines obviously make a good antenna, but a good phone instrument can
eliminate the problem.
If you don't want to replace the phones, a series RF choke (2.5 uH works
well) will eliminate almost every problem on the phone lines, but may limit
or affect the ringing current, depending on the phone. Consider putting one
at the point where the phone enters the house and at each phone.
If you have unused lines, by all means disconnect them at both ends from the
block connectors and let them "float", they can't act as an antenna if
they're not connected.
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