There is a dead cat on the line.
Before the 1970's, telephones out in the country were often on party lines.
If you were on a party line, you heard everybody's phone ring on your phone. You only answered it when it was your particular ring.
Likewise, you could pick up the phone and listen in on what ever was being said by anyone speaking on the phone. It was sometimes easy to know if someone was listening in because if someone picked up the phone after the person you called, there would be a click.
When you heard that click, you might be heard saying, "There is a dead cat on the line." This meant that someone was listening in on your conversation, so be careful what you say.