Strange as it seems, it appears that if you have DSL (internet service that arrives through your telephone line) something similar could be happening. The DSL things (waves? molecules? particles?) don't understand that their only job is to travel efficiently and directly through the wire from the phone jack to the DSL modem. Having DSL means having it ready and available through every phone jack in the house.
I had blamed my lousy phone reception and haphazard, unreliable internet service on a long-ago lightning bolt. Cursed the phone company for not being able to fix it. It was my nephew in Oklahoma who casually asked whether I had a DSL filter.
A what? How is it possible that a little plastic gizmo attached to the end of a phone cord at the OTHER END OF THE HOUSE has made such a dramatic difference? It can only be that the DSL is no longer "leaking" out of that other jack. I'm sure of it.