(CNN) — AT&T says it has resolved an outage that left some customers in the dark on Tuesday.

Earlier, the company said a problem prevented many AT&T customers from completing calls between carriers.

“The interoperability issue between carriers has been resolved,” an AT&T spokesperson said. “We collaborated with the other carrier to find a solution and appreciate our customers patience during this period.”

In a previous statement sent to CNN, the company said a problem prevented many AT&T customers from completing calls between carriers. That also means customers from a rival service could not place calls to an AT&T customer.

Although AT&T did not share the number of impacted customers, website Down Detector shows a spike in reports of issues using the service starting around 1 pm ET. Those numbers climbed in the hours that followed. By 6 pm ET, however, the number of reported incidents started to decline, according to Down Detector.

The site listed New York City, Chicago, Philadelphia, Dallas, Pittsburgh and Indianapolis as among the cities with the most reports of issues.

Because of the interoperability problem, thousands of Verizon customers also reported a service outage on Down Detector Tuesday.

AT&T told CNN that 911 calls went through, despite a few locations, including Camden County, Georgia, and Scranton, Pennsylvania, issuing alerts on social media that the outage was disrupting calls to 911. AT&T told CNN the alerts were received accidentally after a template for such a notification was triggered and sent. AT&T said it is investigating why that happened.

The outage comes less than four months after a massive disruption that knocked out service for AT&T’s network for nearly 12 hours. In February, tens of thousands of AT&T customers in America were unable to make phone calls, send texts, reach emergency services or access the internet because of an AT&T network outage.

In March, the telecommunications company said it had been hacked in a separate incident, and the stolen data contained information such as account holders’ Social Security numbers.

This story has been updated with additional developments and context.

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