The Mystery of the Ghost Calls

We bought a house that that unbeknownst to me, had a second phone line coming in. I had the fax on this line but it was supposed to be on the same line as all the other phones in the house. I figured it out one day when I picked up the fax phone and heard somebody talking. I could not hear them on the other phones in the house. Turns out the ph co. had this line connected to somebodies house down the street!
So maybe make sure all phones lines are really connected to YOUR line.