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

Here’s what to know if you are having problems with your phone service.

When did the outage start?

AT&T had more than 58,000 outages around noon ET, in locations including Houston, Atlanta and Chicago. The outages, which began at approximately 3:30 a.m. ET, peaked at around 73,000 reported incidents. The carrier has more than 240 million subscribers, the country’s largest.

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What is the SOS mode?

Some AT&T iPhone 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, but it can make emergency calls through other carrier networks, according to Apple Support.

What is Wi-Fi calling?

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.

What caused the outage?

AT&T has stayed mum so far on what caused the outage.