The barrage of calls all feature the same short, recorded message: A computerized female voice says the message is a “test call” before twice encouraging people to remain inside. The robocalls, which have come from a slew of fake or unknown numbers, began over the summer and intensified in October, and now appear to have affected nearly every Zip code in the United States.

The reach and timing of the calls recently caught the attention of YouMail, a tech company that offers a robocall-blocking app for smartphones, as well as some of the country’s top telecom carriers, which determined from an investigation that the calls may be foreign in origin and sophisticated in their tactics. Data from YouMail shows that the calls have reached 280 of the country’s 317 area codes since the campaign began in the summer.

While the robocall does not explicitly mention the 2020 presidential election or issues that might affect voters’ well-being, including the coronavirus pandemic, it still threatens to create confusion, said Alex Quilici, YouMail’s chief executive. And it illustrates worrisome vulnerabilities in the country’s phone system, he said, that sophisticated actors could exploit.