And per a new Federal Communications Commission report, the root cause of the crisis—which is not believed to have led to any deaths—was very simple. From the Washington Post:

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.