Several phone service providers, including AT&T, were down for many users across the United States early Thursday, according to Downdetector.com, which tracks telecommunication and internet disruptions.

The outage, which affected users in cities including Atlanta, Los Angeles and Dallas, was first reported around 3:30 a.m. Eastern time, and were still being widely reported four hours later.

AT&T did not immediately respond to a request for comment. The company said on its website that there were also wireless outages in San Diego, Richmond and Milwaukee, with the initial cause listed as “maintenance activity.”

As of 7:30 a.m., about 50,000 people were reporting to Downdetector that they were experiencing problems with AT&T. Verizon and T-Mobile were seeing much smaller numbers of reports, at about 3,000 and 1,000, respectively.

The San Francisco Fire Department said on social media that it was aware of an issue impacting AT&T users who were trying to call 911. “We are actively engaged and monitoring this,” the fire department said. “If you are an AT&T customer and cannot get through to 911, then please try calling from a landline.”

Outages on a smaller scale were reported by some users of Cricket Wireless, Verizon, FirstNet and T-Mobile.

In an e-mail, T-Mobile said: “We did not experience an outage. Our network is operating normally. Downdetector is likely reflecting challenges our customers were having attempting to connect to users on other networks.”

This is a developing story.