Some residents and businesses in rural America face a perplexing problem: They know people are trying to call their phones and sometimes failing, but they don’t know why.

“Our customers can call out to the world just fine,” said Scott Behn, chief executive of Mosaic Telecom, a local cooperative that provides phone service to about 7,500 subscribers around Barron County in Wisconsin. “It’s customers from around the world or sometimes the next town over trying to call in. These types of calls never hit our switch.”