Customers of numerous U.S. cell service providers were reporting outages early Thursday morning, according to the website Downdetector.com, which tracks such reports.
AT&T had by far the most, with tens of thousands of customers telling Downdetector they had no service.
The reports of AT&T outages appear to have started at around 4 a.m. EST, Downdetector said.
It seemed to be waning some two hours later, then picked up again and were topping 60,000 at 7:30 a.m. EST.
Some municipalities were experiencing difficulties with 911 service.
The San Francisco Fire Department said in a post on X, the former Twitter, "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. 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. Do not call or text 911 to simply test your phone service."
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 cell phone 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."
There was no word on the source of AT&T's problems.
CBS News said it had reached out to all the major carriers for comment.