AT&T wireless customers in cities across the country lost service overnight, causing frustration and concern about disruptions to 911 dispatches.

Around 3:30 a.m. ET Thursday, outages reported by downdetector.com suddenly spiked from just a handful to more than 71,000 by around 9 a.m. ET.

In a statement emailed to NPR, AT&T said: "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."

The website said that Houston, Dallas, San Antonio, Chicago, Indianapolis, New York, Miami and Charlotte reported the highest number of outages. Houston had more than 2,000 reports by about 8:30 a.m. ET, while New York reported about 1,300.

However, service disruptions caused concern even outside those areas, with the San Francisco Fire Department announcing on X, formerly Twitter, that it was "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," the department said.