Verizon Communications’ mobile network was down for thousands of users in the United States on Monday, according to outage tracking website Downdetector.com, leaving those with “SOS” at the top of their phone wondering what was going on.

What does “SOS” mean on your phone?

According to Apple’s support center, if you see “No Service” or “Searching” in the status bar on your iPhone or iPad, your device isn’t connected to a cellular network. If you see “SOS” or “SOS only” in the status bar, your device can still make emergency calls.

This feature is available in the U.S., Canada and Australia.

People with an iPhone 13 or earlier versions won’t get the SOS indicator but will see a “No service” indicator, instead.

Does being on Wi-Fi change anything?

The SOS mode is indicative of your cellular service, so being able to connect to Wi-Fi could help you be able to send texts, access the internet and make calls.

What’s happening?

The outages began at around 8:30 a.m. Dallas time, Downdetector said, with regions including Minneapolis, Phoenix, Omaha and Denver among the most reported locations.

Verizon said it was aware of an issue impacting its services. “Our engineers are engaged and we are working quickly to identify and solve the issue.”

Reuters wire service contributed to this report.