The April 9 outage actually affected 11 million people across seven states. According to the Washington Post, close to six thousand people tried to call 911 but were unable to complete the call.
At the center of the disruption was a system maintained by a third-party contractor, a Colorado-based company called Intrado. Intrado owns and operates a routing service, taking in 911 calls and directing them to the most appropriate public safety answering point, or PSAP, in industry parlance. Ordinarily, Intrado's automated system assigns a unique identifying code to each incoming call before passing it on — a method of keeping track of phone calls as they move through the system.
But on April 9, the software responsible for assigning the codes maxed out at a pre-set limit; the counter literally stopped counting at 40 million calls. As a result, the routing system stopped accepting new calls, leading to a bottleneck and a series of cascading failures elsewhere in the 911 infrastructure.
Intrado employees reportedly failed to notice the warning alerts and categorized them as "low level" issues.
The company says it has corrected the issue and created a new warning alarm to signal when the number of successfully routed calls drops below a certain level.