Millions of AT&T customers across the US were unable to make or receive calls, send texts, or access the internet for much of Thursday because of a major outage that started Thursday morning, and we finally know why.

Thursday evening, AT&T said, "Based on our initial review, we believe that today's outage was caused by the application and execution of an incorrect process used as we were expanding our network, not a cyber attack."   

According to tracking site Downdetector, outages were first reported around 3:30 a.m. ET. Customers attempting to place a call were greeted with an error message saying "Cellular network not available." 

AT&T is the largest telecommunications company in the United States, accounting for a 47% market share of wireless subscriptions, or 241.5 million customers. 

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While millions of customers couldn't text, call, or access 5G internet, some AT&T customers reported having text access.

After service was restored in the early afternoon, AT&T said: "We have restored wireless service to all our affected customers. We sincerely apologize to them. Keeping our customers connected remains our top priority, and we are taking steps to ensure our customers do not experience this again in the future."

Enabling Wi-Fi calling reportedly fixed the problem for some, but many users (including myself) were greeted with a spinning wheel and an eventual error message when they tried to turn on this feature, leaving them disconnected still. 

The biggest concern was of course 911 emergency services. If you have text access, 10 states -- Arizona, Connecticut, Delaware, Indiana, Iowa, Minnesota, Maine, New Hampshire, North Dakota, and South Dakota -- do allow citizens to text 911 in an emergency. 

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Some Verizon and T-Mobile customers also reported issues, but those companies said their networks were fine, and these issues were most likely tied to customers attempting to reach someone on the AT&T network.

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