911 dispatcher takes calls A 911 dispatcher takes calls. David Kidd

Last month, dispatch centers in Oregon and Washington were unable to receive incoming 911 calls for several hours, and now the cause has been revealed: An estimated 4,500 calls meant for these state dispatch centers got stuck at a CenturyLink processing center in Englewood, Colo.

According to an April 24 report, CenturyLink routes 911 calls to the appropriate state-run dispatch centers using a third-party vendor called Intrado. To accomplish this, Intrado’s Colorado complex assigns each call a unique identifying key, which allows the call to be forwarded on. On April 10, however, they ran out of keys.

Usually, Intrado periodically purges its system to make room for new calls and ensure there are always enough keys. But for whatever reason, this had not been done since the previous September, according to the News Tribune, and the result was a lapse in service for Washington and Oregon dispatch centers. Somewhere between 40 million and 60 million keys were used after the September purge occurred.

On April 10, it took several hours to identify the problem, because running out of keys was classified as a minor issue, and therefore did not trigger a major alarm. It wasn't until the state dispatch centers called into the CenturyLink processing center that the processing center realized something was wrong.

A report revealed that Minnesota and North Carolina dispatch centers also experienced a lapse in service, with the number of missed calls unknown. While 127 call centers in Washington state were affected, in Minnesota the number was between six and eight, and in North Carolina it was about 11, Andy Leneweaver, 911 project manager, told the News Tribune.

The problem has reportedly been fixed as the threshold for unique call keys that can be issued was raised to 8 billion, and an employee has been assigned to actively monitor how many keys are left available. CenturyLink told Washington state in its report that, "... it is not theoretically possible to exhaust the threshold ranges.”