(Credit: Brett Coomer/Houston Chronicle/Getty Images via CNN Newsource)

(CNN) — AT&T’s network went down for many of its customers across the United States Thursday morning, leaving customers unable to place calls, text or access the internet. By late morning, the company said most of its network had been restored.

Although Verizon and T-Mobile customers reported some network outages, too, they appeared far less widespread. T-Mobile and Verizon said their networks were unaffected by AT&T’s service outage and customers reporting outages may have been unable to reach customers who use AT&T.

More than 74,000 AT&T customers reported outages on digital-service tracking site DownDetector, with service disruptions beginning around 4 am ET. That’s not a comprehensive number: It tracks only self-reported outages. Reports had been rising steadily throughout the morning but leveled off in the 9 am ET hour. By 11 am ET, the DownDetector data showed some 60,000 AT&T customers still reporting outages.

AT&T acknowledged that it has a widespread outage but did not provide a reason for the system failure.

“Some of our customers are experiencing wireless service interruptions this morning. We are working urgently to restore service to them,” AT&T said in a statement. “We encourage the use of Wi-Fi calling until service is restored.”

To set up Wi-Fi calling, users can go to their Settings app on their phone. iPhone users should tap “Cellular” and Android users should click “Connection” and then users will be prompted to turn on the Wi-Fi calling feature. AT&T says on its website that there is no extra cost for this feature. Once set up, Wi-Fi calling works automatically when you’re connected to a Wi-Fi that you choose.

By late morning, AT&T said most of its network was back online.

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

The company did not have a timeframe for when its system would be fully restored. AT&T has been responding to customer complaints online, asking them to send direct messages to customer service.

Why AT&T Went Down

AT&T has encountered sporadic outages over the past few days, including a temporary 911 outage in some parts of the southeastern United States. Although outages happen from time to time, nationwide, prolonged outages are exceedingly rare.

Although AT&T provided no official reason for the outage, the issue appears to be related to how cellular services hand off calls from one network to the next, a process known as peering, according to an industry source who spoke on the condition of anonymity.

There’s no indication that Thursday’s outage was the result of a cyberattack or other malicious activity, the industry source said.

Verizon believes the nationwide outage involving AT&T customers “is close to being resolved,” according to Richard Young, a Verizon spokesman.

Carriers are notoriously mum about why their networks go down. In the past, there have been construction accidents that have cut fiberoptic cables, incidents of sabotage or network updates filled with bugs that became difficult to roll back.

Local Governments Report Outages

Several local governments said AT&T’s outage was disrupting its services.

The city of Chicago’s Office of Emergency Management said it is monitoring the situation and the 911 center remained operational. 

Verizon and T-Mobile Say They’re Unaffected

There also have been about 1,000 outages reported by both Verizon and T-Mobile customers Thursday morning, the DownDetector website indicates.

“We did not experience an outage,” T-Mobile said in a statement. “Our network is operating normally.”

Verizon had a similar comment, saying it was unaffected by AT&T’s outage.

“Verizon’s network is operating normally,” Verizon told CNN in a statement. “Some customers experienced issues this morning when calling or texting with customers served by another carrier. We are continuing to monitor the situation.”

User reports on Downdetector about a T-Mobile outage, the company added, are “likely reflecting challenges our customers were having attempting to connect to users on other networks.”

Downdetector offers “real-time status information for over 12,000 services across 47 websites representing 47 countries,” the website says.