A local council estate where I used to live had a door to door salesman doing the rounds selling wireless door bells when they first came out. Relatively cheap and easy to install, many of the residents bought and installed them only to find out that they were all on the same radio frequency, so if one bell push was pressed, every bell within range rang!
The Rhyme of the ancient Mariner by Iron Maiden, 15 or so minutes long, anyne who waits that long is either family or really really desperate to sell you something.