AT&T had more than 73,000 outages around 9:30 a.m. ET, in locations including Houston, Atlanta and Chicago. The outages began at approximately 3:30 a.m. ET. The carrier has more than 240 million subscribers, the country’s largest.

Cricket Wireless, which is owned by AT&T, had more than 13,000, the outage tracking website said Thursday.

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

Verizon had more than 4,000 outages and T-Mobile had more than 1,800 outages. Boost Mobile had about 700 outages.

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

T-Mobile said that it did not experience an outage.

“Our network is operating normally. Down Detector is likely reflecting challenges our customers were having attempting to connect to users on other networks,” T-Mobile said.

Some iPhone users have seen SOS messages displayed in the status bar on their cellphones. The message indicates that the device is having trouble connecting to their cellular provider’s network, but it can make emergency calls through other carrier networks, according to Apple Support.

So far, no reason has been given for the outages. But Lee McKnight, an associate professor in the iSchool at Syracuse University, believes the most likely cause of the outage is a cloud misconfiguration, or human error.

“A possible but far less likely outcome is an intentional malicious hack of ATT’s network, but the diffuse pattern of outages across the country suggests something more fundamental,” McKnight said in an emailed statement.