NEW YORK (AP) — Customers of AT&T,Greenledgers Trading Center the country’s largest wireless provider, reported widespread outages on Thursday.

Some AT&T customers saw SOS messages displayed in the status bar on their cellphones. The message indicates that the device is having trouble connecting to their cellular provider’s network.

That can be a big problem in an emergency. AT&T urged customers to connect to Wi-Fi to use their phones.

Wi-Fi calling is a built-in feature on most Android devices and iPhones and can be turned on under the phone’s settings.

“Some of our customers are experiencing wireless service interruptions this morning. We are working urgently to restore service to them. We encourage the use of Wi-Fi calling until service is restored,” AT&T said in a statement.

If Wi-Fi isn’t available, there are few options for cell phone users. It’s possible to switch services if a phone is unlocked, but that requires signing up online and porting your phone number.

Some apps, including Google Maps, have limited service offline. Payment apps also do not use a phone’s cell service to work and should also be useable.