AT&T tweeted around 10:30 p.m.
Aware of issue affecting some calls to 911 for wireless customers.
AT&T had about 135 million wireless subscribers in the United States as of December 31, 2016, according to a company filing.
AT&T customers in North Texas are experiencing difficulty placing 911 emergency calls. "We apologize to those affected", an AT&T spokesman said in a statement.
Austin and Travis County AT&T customers should call 311 in case of an emergency until further notice, the Austin Police Department and Travis County Sheriff's Office said. However, the company did not offer an explanation for why the issue occurred in the first place.
AT&T users throughout Florida may not be able to call 911.
The Federal Communications Commission is investigating what caused a 911 outage that affected AT&T wireless customers in multiple states on Wednesday.
Houston Police Chief Art Acevedo also acknowledged the issue on Twitter.
The 911 outage appears to be statewide, Seminole County officials said earlier Wednesday night.
The issue was not caused by 911 failures, but by as-yet unknown or unexplained AT&T problems. If you call 911 for a non-emergency matter, it will not result in a faster response. Service has since been restored, but given the seriousness of the situation, the FCC has opened an investigation into what went wrong.