Beginning this morning, AT&T's cellular network suffered a major outage across the US. At this point, AT&T has not made any statement as to the nature of the outage. It is far too early to speculate. In the past, similar outages were often caused by misconfigurations or technology failures.

What makes the outage specifically significant is that phones cannot connect to cell towers in some areas. This means you cannot make any calls, send or receive SMS messages, or reach emergency services. Some iPhones display the "SOS" indicator, which is displayed if the phone is able to make emergency-only calls via another provider's network. For some newer iPhones, satellite services may be used.

As a workaround, WiFi calling is still reported to work. If you do have an internet connection via a different provider and are able to enable WiFi calling, you will be able to make and receive calls.

Note that this will affect many devices like location trackers, alarm systems, or other IoT devices that happen to use AT&T's network. This could have security implications if you rely on these devices to monitor remote sites.

Some users are reporting that service has already been restored in their area, but without an official statement from AT&T, it is hard to predict how long service restoration will take. For your own planning purposes, I would assume it will be several hours for the service to be fully restored.

Some other workarounds to consider:

• Many modern phones will allow for a second eSIM to be installed. T-Mobile and WiFi sometimes offer free trials of their network, and it may get you going for the day.
• As mentioned above, WiFi calling is reported to work.
• Get a data-only eSIM with a low-cost provider with enough data to "survive" the day.

For resiliency, it is always best to have a secondary internet connection available. In many cases, the cellular connection is your secondary connection. The outage also effects AT&T resellers (MVNOs) like Cricket, Consumer Celular, and Straight Talk.

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Johannes B. Ullrich, Ph.D. , Dean of Research, SANS.edu
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