What is line-trapping? The article doesn't explain how it works and which devices are vulnerable. I tried to look it up but the results aren't helpful. They are

What is line-trapping? The article doesn't explain how it works and which devices are vulnerable. I tried to look it up but the results aren't helpful. They are either rewrites of the same article or irrelevant pages about wildlife trapping. I suspect they affect landlines, but the articles have confusing photos showing smartphones.