Several cell service providers suffered unexpected outages Thursday morning, leaving tens of thousands of customers across the U.S. unable to make calls, send texts or access the internet.

AT&T’s network reported the most outages, with over 74,000 customers reporting a lack of service to Downdetector as of 6:30 a.m. MST. According to the website, cell signal is the major concern, although 8% of reports mentioned issues with mobile internet.

NBC reports that Cricket Wireless, which is owned by AT&T, is facing similar outages.

Several thousand Verizon and T-Mobile customers also reported service issues, although representatives from both companies told Axios their networks were operating normally and that the issues likely stemmed from customers attempting to contact users on other carriers.

AT&T is aware of the issue and is working to resolve it. A statement posted on the company’s website at 9:15 a.m. MST said it successfully restored the majority of its network.

“Our network teams took immediate action and so far three-quarters of our network has been restored,” the statement reads. “We are working as quickly as possible to restore service to remaining customers.”

The cause of Thursday’s cell service outage has not been publicly identified. The U.S. Cybersecurity and Infrastructure Security Agency told NBC it is in touch with AT&T and ready to offer assistance.

According to Downdetector, the highest concentration of AT&T outage reports came from users in Houston, Chicago, Dallas, San Antonio, Atlanta, New York City, Austin, Miami and Indianapolis. The majority of Verizon and T-Mobile’s outages have been reported in the same cities.

Thursday’s phone issues prompted several safety warnings from emergency services even in areas the outage didn’t widely affect. The San Francisco Fire Department issued a warning to local residents on social media.

“We are aware of an issue impacting AT&T wireless customers from making and receiving any phone calls (including to 911),” the department’s post said. “If you are an AT&T customer and cannot get through to 911, then please try calling from a landline. If that is not an option then please try to get ahold of a friend or family member who is a customer of a different carrier and ask them to call 911 on your behalf.”

As of 2022, less than a quarter of Americans had landline phones in their homes, according to The Washington Post.

If you need to contact emergency services but do not have cell service or a landline, internet service is your best bet. If your phone is connected to the internet, you can enable Wi-Fi calling. If you have access to a computer, you can contact 911 via Skype.