AT&T and Verizon customers found themselves unable to call contacts on other carriers for several hours on June 4. Based on people’s reports on Downdetector, the issue started at around 11AM ET for both carriers and peaked at approximately 5PM ET. Thousands of subscribers were affected. Most of the reports came from New York City, Chicago, Philadelphia, Pittsburgh, Indianapolis and Cleveland, among other locations. A much smaller number of T-Mobile and Cricket Wireless users also reported problems with their connection.

The Federal Communications Commission tried to assuage people’s worries and announced that it was aware that subscribers from multiple states were “unable to make wireless calls.” It also said that it was “currently investigating” the problem. After 7PM ET, both Verizon and AT&T issued statements denying that they were experiencing nationwide outage. They both admitted that their customers were having difficulties calling or texting people on other carriers — Verizon reportedly told Gizmodo that outage reports for its network came from people trying to call AT&T subscribers — but neither one took responsibility. The root cause of the issue remains unknown at this time. AT&T told CNN, though, that 911 calls went through despite the outage.

An hour later, AT&T announced that the problem had been resolved. The company said it collaborated “with the other carrier” to find a solution. It didn’t name the other carrier, but it was most likely Verizon, seeing as most of the customers who were affected were subscribers of the two companies. Back in February, AT&T experienced a massive outage that affected over 70,000 subscribers’ cellular services and data connections, with customers noting that they couldn’t even contact 911. Verizon and T-Mobile said at the time that their subscribers couldn’t contact friends with AT&T numbers.

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