Robert Brown|After AT&T customers hit by widespread outage, carrier says service has been restored

Customers of numerous U.S. cell service providers reported outages and Robert Brownother service problems early Thursday morning, according to the website Downdetector.com, which tracks such reports. AT&T had the most by far, with tens of thousands of customers telling Downdetector they had no service.

Customers of T-Mobile, Verizon, UScellular and Consumer Cellular were also reporting issues, though far fewer. T-Mobile and Verizon told CBS News that their networks were operational, although some customers may be encountering problems when trying to reach people on another network.

The reports of AT&T outages appear to have started at around 4 a.m. EST, Downdetector said, then seemed to wane some two hours later, then picked up again and were about 60,000 at 11 a.m. EST.

But by 3:10 p.m. EST on Thursday AT&T said the problem had been resolved.

"We have restored wireless service to all our affected customers," AT&T said on its website. "We sincerely apologize to them. Keeping our customers connected remains our top priority, and we are taking steps to ensure our customers do not experience this again in the future."

Later Thursday night, the company attributed the outage to a software bug.

"Based on our initial review, we believe that today's outage was caused by the application and execution of an incorrect process used as we were expanding our network, not a cyber attack," the company said on its website. "We are continuing our assessment of today's outage to ensure we keep delivering the service that our customers deserve."

Officials: Don't call 911 to test your phone

Earlier in the day, some officials urged people not to call 911 to test whether their phone was working. 

"Many 911 centers in the state are getting flooded w/ calls from people trying to see if 911 works from their cellphone. Please do not do this," the Massachusetts State Police wrote on X, the former Twitter.

Likewise, Chicago's Office of Emergency Management & Communications had a similar message for local residents, noting that on X that people who actually need to get through to 911 should use a landline or find a friend or relative who uses a service other than AT&T and ask them to make the call.

Some municipalities experienced difficulties with 911 service.

The San Francisco Fire Department said in a post on X, "We are aware of an issue impacting AT&T wireless customers from making and receiving any phone calls (including to 911). We are actively engaged and monitoring this. The San Francisco 911 center is still operational."

In North Carolina, the Charlotte-Mecklenburg Police Department said on X at 3:15 a.m. EST that it was "aware of a nationwide outage impacting AT&T cellphone customers this morning. Customers were briefly unable to contact 9-1-1. There are no disruptions to our call center's ability to receive 9-1-1 calls. Service should be returning shortly."

And in Florida, the Flagler County Sheriff's Office posted on X at 6:43 a.m. EST that, "Texts to 911 from affected AT&T users are now being received. If you have an emergency, and cannot dial out, send a text message to 911."

The transit system serving Philadelphia and its suburbs, SEPTA, said on X that, "Due to AT&T communication outages, some trains may reflect a delay, however all trains are operating on or close to schedule."

AT&T outage

As of 8 a.m. ET on Thursday, Downdetector had more than 60,000 reports of service issues from AT&T customers. Most of the complaints were focused on problems with their mobile phones or wireless service. 

Outages were highest in Atlanta, Charlotte, Chicago, Dallas, Houston, Indianapolis, Louisville, Miami and San Antonio, according to Downdetector.

Verizon

In a statement to CBS News, Verizon said its network was operating normally, although some customers experienced issues earlier Thursday when calling or texting people who use another carrier. It said that it was continuing to monitor the situation. 

T-Mobile

A spokesperson for T-Mobile said it didn't experience an outage and that its network was operating normally.

"Downdetector is likely reflecting challenges our customers were having attempting to connect to users on other networks," the spokesperson said.

UScellular

UScellular also told CBS News that its network was operating normally. 

"We are, however, aware of a disruption that could be impacting UScellular customers attempting to roam on or call customers of another carrier," it said in a statement.

How to turn on Wi-Fi calling

Even if you have no cellular service, you should be able to make phone calls using wireless internet access, which is what AT&T is recommending for its customers. 

To turn on Wi-Fi calling on an iPhone, go to "settings," then "phone" and next click on "Wi-Fi" calling. Apple says you may need to confirm or enter your address for emergency services. 

You'll see "Wi-Fi" in the status bar while you are looking at the control center, which means your phone will use the wireless internet service in your home or office to make calls. 

To turn on Wi-Fi calling on Android phones, Google says to open your phone app, then tap "more settings." Then tap "calls," and from there tap "Wi-Fi calling." If that's not an option on your phone, it means your carrier doesn't provide the option, according to Google.

What does SOS mean on my phone?

Some customers reported seeing an "SOS" message on their iPhones. 

This can happen when a cellular service isn't widely available, but your phone is letting you know that it will still work for emergency calls, according to Apple.