AT&T on Tuesday said the telecommunications company and other wireless carriers are working to fix an issue preventing customers from completing calls placed to non-AT&T customers across the U.S.

"There is a nationwide issue that is affecting the ability of customers to complete calls between carriers. The carriers are working as quickly as possible to diagnose and resolve the issue," the carrier stated in a post on X.

Calls between AT&T customers were not impacted, the telecom said.

The website Downdetector showed a surge in customer reports of trouble using AT&T's service beginning about 1 p.m. Eastern Time, with the numbers rising during the following few hours. The site showed Chicago, Dallas, Indianapolis, New York City, Philadelphia and Pittsburgh as among the cities most impacted. 

Downdetector also showed a spike of outage reports by Verizon customers. Although the wireless carrier said its network was "operating normally," the company also noted in a statement that some customers, "primarily in the Northeast and Midwest, are experiencing issues when calling or texting with customers served by another carrier. We are continuing to monitor the situation."

AT&T's customer support service was deluged with complaints. Some AT&T users, including Virginia's Hanover County and Georgia's Camden County, also posted alerts on social media stating that the AT&T outage was affecting emergency calls to 911. 

"Nationwide 911 services are operating normally at this time and our customers are not affected. We're working to correct a wireless impact notification that was sent in error to 911 call centers," a spokesperson for AT&T said.

The outages come after millions of current and former AT&T customers learned last month that hackers likely stole their personal information and were sharing it on the "dark web". The massive data breach was the latest after another leak impacting 9 million users early last year.